diff --git a/Cargo.lock b/Cargo.lock
index 5c310ac..088597e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -28,7 +28,7 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "daisycalc"
-version = "0.2.5"
+version = "0.2.8"
 dependencies = [
  "cfg-if",
  "rug",
diff --git a/Cargo.toml b/Cargo.toml
index b1fec27..9104008 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "daisycalc"
-version = "0.2.5"
+version = "0.2.8"
 edition = "2021"
 build = "buildscript/main.rs"
 license-file = "LICENSE"
diff --git a/TODO.md b/TODO.md
index e71e136..e0bb361 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,14 +1,11 @@
- - Fix unit autoconversion (Hz * s)
  - Minimize parenthesis when printing expressions
- - Sane autoconversion (mi + km)
  - Consistent errors:
    - Consistent look
-   - Linelocation for math errors
    - Undefined error?
    - Better error when `sin = 2`
    - Check syntax when parsing or when evaluating?
  - list and delete variables
- - redefine variables
+ - sin() with units (rad and deg)
 
 ## Pre-release
  - Commands to list constants, units, and substitutions
diff --git a/src/command/mod.rs b/src/command/mod.rs
index 5e0d345..e55c6dd 100644
--- a/src/command/mod.rs
+++ b/src/command/mod.rs
@@ -134,6 +134,9 @@ pub fn do_command(
 					"  hyperbolic sin, etc      {c}sinh, asinh, csch{r}\r\n",
 					"  hyperbolic cos, etc      {c}cosh, acosh, sech{r}\r\n",
 					"  hyperbolic tan, etc      {c}tanh, atanh, coth{r}\r\n",
+					"\r\n",
+					"  convert to base unit     {c}tobase(quantity){r}\r\n",
+					"  remove units             {c}nounit(quantity){r}\r\n",
 					"\n"
 				),
 
@@ -142,7 +145,7 @@ pub fn do_command(
 				t = format!("{}{}", color::Fg(color::Magenta), style::Bold)
 			)?;
 		},
-		_ => panic!("Bad command!")
+		_ => unreachable!("Bad command!")
 	};
 
 	return Ok(());
diff --git a/src/context.rs b/src/context.rs
index f6d3af1..f44d8be 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -13,19 +13,29 @@ impl Context {
 	}
 
 	pub fn push_hist(&mut self, t: Expression) { self.history.push(t); }
-	pub fn push_var(&mut self, s: String, t: Expression) { self.variables.insert(s, t); }
+	pub fn push_var(&mut self, s: String, t: Expression) -> Result<(), ()> {
+		if self.valid_varible(&s) {
+			self.variables.insert(s, t);
+			return Ok(());
+		} else { return Err(()); }
+	}
 	pub fn del_var(&mut self, s: &String) { self.variables.remove(s); }
 
 	pub fn get_variable(&self, s: &String) -> Option<Expression> {
-
 		let v: Option<&Expression>;
-
 		if s == "ans" {
 			v = self.history.last();
 		} else {
 			v = self.variables.get(s);
 		}
-
 		if v.is_some() { Some(v.unwrap().clone()) } else { None }
 	}
+
+	pub fn valid_varible(&self, s: &str) -> bool {
+		return match s {
+			"ans" => false,
+			_ => true
+		}
+	}
+
 }
diff --git a/src/entry/mod.rs b/src/entry/mod.rs
index 798a76c..10003b7 100644
--- a/src/entry/mod.rs
+++ b/src/entry/mod.rs
@@ -5,8 +5,7 @@ cfg_if::cfg_if! {
 		pub use unix::main as main_e;
 	} else {
 		pub fn main_e () -> Result<(), std::io::Error> {
-			println!("Not yet implemented.");
-			Ok(())
+			unimplemented!("Not yet implemented.");
 		}
 	}
 }
\ No newline at end of file
diff --git a/src/entry/unix/unix.rs b/src/entry/unix/unix.rs
index 35b7335..bf7b400 100644
--- a/src/entry/unix/unix.rs
+++ b/src/entry/unix/unix.rs
@@ -59,24 +59,24 @@ fn do_expression(
 	let Ok(g) = g else {panic!()};
 
 
-	// Display parsed string
-	write!(
-		stdout, " {}{}=>{}{} {}\r\n",
-		style::Bold, color::Fg(color::Magenta),
-		style::Reset, color::Fg(color::Reset),
-		g.to_string()
-	).unwrap();
-
 
 	// Evaluate expression
 	#[cfg(debug_assertions)]
 	RawTerminal::suspend_raw_mode(&stdout).unwrap();
-	let g = evaluate(&g, context);
+	let g_evaluated = evaluate(&g, context);
 	#[cfg(debug_assertions)]
 	RawTerminal::activate_raw_mode(&stdout).unwrap();
 
 	// Show output
-	if let Ok(q) = g {
+	if let Ok(q) = g_evaluated {
+		// Display parsed string
+		write!(
+			stdout, " {}{}=>{}{} {}\r\n",
+			style::Bold, color::Fg(color::Magenta),
+			style::Reset, color::Fg(color::Reset),
+			g.to_string()
+		).unwrap();
+
 		write!(
 			stdout, "\n  {}{}={} {}{}\r\n\n",
 			style::Bold,
@@ -88,12 +88,22 @@ fn do_expression(
 		return Ok(q);
 
 	} else {
-		match g {
-			Ok(_) => panic!(),
+		match g_evaluated {
+			Ok(_) => unreachable!(),
 
-			Err(EvalError::TooBig) => {
+			Err((l, EvalError::TooBig)) => {
 				write!(
-					stdout, "\n  {}{}Mathematical Error: {}Number too big{}\r\n\n",
+					stdout, "{}{}{}{}{}{}\r\n",
+					color::Fg(color::Red),
+					style::Bold,
+					" ".repeat(l.pos + 4),
+					"^".repeat(l.len),
+					color::Fg(color::Reset),
+					style::Reset,
+				).unwrap();
+
+				write!(
+					stdout, "  {}{}Mathematical Error: {}Number too big{}\r\n\n",
 					style::Bold,
 					color::Fg(color::Red),
 					style::Reset,
@@ -101,9 +111,19 @@ fn do_expression(
 				).unwrap();
 			},
 
-			Err(EvalError::ZeroDivision) => {
+			Err((l, EvalError::ZeroDivision)) => {
 				write!(
-					stdout, "\n  {}{}Mathematical Error: {}Division by zero{}\r\n\n",
+					stdout, "{}{}{}{}{}{}\r\n",
+					color::Fg(color::Red),
+					style::Bold,
+					" ".repeat(l.pos + 4),
+					"^".repeat(l.len),
+					color::Fg(color::Reset),
+					style::Reset,
+				).unwrap();
+
+				write!(
+					stdout, "  {}{}Mathematical Error: {}Division by zero{}\r\n\n",
 					style::Bold,
 					color::Fg(color::Red),
 					style::Reset,
@@ -111,9 +131,19 @@ fn do_expression(
 				).unwrap();
 			},
 
-			Err(EvalError::BadMath) => {
+			Err((l, EvalError::BadMath)) => {
 				write!(
-					stdout, "\n  {}{}Mathematical Error: {}Failed to evaluate expression{}\r\n\n",
+					stdout, "{}{}{}{}{}{}\r\n",
+					color::Fg(color::Red),
+					style::Bold,
+					" ".repeat(l.pos + 4),
+					"^".repeat(l.len),
+					color::Fg(color::Reset),
+					style::Reset,
+				).unwrap();
+
+				write!(
+					stdout, "  {}{}Mathematical Error: {}Failed to evaluate expression{}\r\n\n",
 					style::Bold,
 					color::Fg(color::Red),
 					style::Reset,
@@ -121,9 +151,19 @@ fn do_expression(
 				).unwrap();
 			},
 
-			Err(EvalError::IncompatibleUnit) => {
+			Err((l, EvalError::IncompatibleUnit)) => {
 				write!(
-					stdout, "\n  {}{}Evaluation Error: {}Incompatible units{}\r\n\n",
+					stdout, "{}{}{}{}{}{}\r\n",
+					color::Fg(color::Red),
+					style::Bold,
+					" ".repeat(l.pos + 4),
+					"^".repeat(l.len),
+					color::Fg(color::Reset),
+					style::Reset,
+				).unwrap();
+
+				write!(
+					stdout, "  {}{}Evaluation Error: {}Incompatible units{}\r\n\n",
 					style::Bold,
 					color::Fg(color::Red),
 					style::Reset,
@@ -131,9 +171,19 @@ fn do_expression(
 				).unwrap();
 			},
 
-			Err(EvalError::BadDefineName) => {
+			Err((l, EvalError::BadDefineName)) => {
 				write!(
-					stdout, "\n  {}{}Evaluation Error: {}Invalid variable name{}\r\n\n",
+					stdout, "{}{}{}{}{}{}\r\n",
+					color::Fg(color::Red),
+					style::Bold,
+					" ".repeat(l.pos + 4),
+					"^".repeat(l.len),
+					color::Fg(color::Reset),
+					style::Reset,
+				).unwrap();
+
+				write!(
+					stdout, "  {}{}Evaluation Error: {}Invalid variable name{}\r\n\n",
 					style::Bold,
 					color::Fg(color::Red),
 					style::Reset,
diff --git a/src/evaluate/evaluate.rs b/src/evaluate/evaluate.rs
index cc5cdb1..54b2456 100644
--- a/src/evaluate/evaluate.rs
+++ b/src/evaluate/evaluate.rs
@@ -1,13 +1,13 @@
 use crate::parser::Expression;
 use crate::parser::Operator;
 use crate::context::Context;
-
+use crate::parser::LineLocation;
 
 use super::operator::eval_operator;
 use super::function::eval_function;
 use super::EvalError;
 
-pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, EvalError> {
+pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, (LineLocation, EvalError)> {
 
 	// Keeps track of our position in the expression tree.
 	// For example, the coordinates [0, 2, 1] are interpreted as follows:
@@ -40,15 +40,23 @@ pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, Eva
 
 
 			let new = match g {
-				Expression::Quantity(_) => None,
+				Expression::Quantity(_, _) => None,
 
-				Expression::Constant(c) => { Some(evaluate(&c.value(), context).unwrap()) },
-				Expression::Variable(s) => { context.get_variable(&s) },
-				Expression::Operator(Operator::Function(f), v) => { Some(eval_function(&f, &v)?) },
-				Expression::Operator(o, v) => { eval_operator(&o, &v, context)? },
+				Expression::Constant(_, c) => { Some(evaluate(&c.value(), context).unwrap()) },
+				Expression::Variable(_, s) => { context.get_variable(&s) },
+				Expression::Operator(_, Operator::Function(_), _) => { Some(eval_function(g)?) },
+				Expression::Operator(_, _, _) => { eval_operator(g, context)? },
 			};
 
-			if new.is_some() { *g = new.unwrap()}
+			if let Some(mut new) = new {
+				if let Expression::Constant(_,_) = g {
+					// Fix constant line location.
+					// Constant expansion does not change the location of a value,
+					// but other operations might.
+					new.set_linelocation(&g.get_linelocation());
+				}
+				*g = new;
+			}
 
 
 			// Move up the tree
@@ -59,14 +67,16 @@ pub fn evaluate(t: &Expression, context: &mut Context) -> Result<Expression, Eva
 
 		} else {
 			// Move down the tree
-			coords.push(0);
 
-			let n = root.get_at_coords(&coords[..]);
-			if let Some(n) = n {
-				if let Expression::Operator(Operator::Define, _) = n {
-					*coords.last_mut().unwrap() += 1;
-				}
+			// Don't evaluate the first argument of a define.
+			// This prevents variables from being expanded before a re-assignment.
+			if let Expression::Operator(_, Operator::Define, _) = g {
+				*coords.last_mut().unwrap() += 1;
+				coords.push(0);
+				continue;
 			}
+
+			coords.push(0);
 		}
 	}
 
diff --git a/src/evaluate/function.rs b/src/evaluate/function.rs
index 11d58b1..73c9e61 100644
--- a/src/evaluate/function.rs
+++ b/src/evaluate/function.rs
@@ -1,59 +1,61 @@
-use std::collections::VecDeque;
-
 use crate::parser::Expression;
 use crate::parser::Function;
+use crate::parser::Operator;
+use crate::parser::LineLocation;
 use super::EvalError;
 
+pub fn eval_function(g: &Expression) -> Result<Expression, (LineLocation, EvalError)> {
+
+	let Expression::Operator(loc, Operator::Function(f), args) = g else {panic!()};
 
-pub fn eval_function(f: &Function, args: &VecDeque<Expression>) -> Result<Expression, EvalError> {
 	if args.len() != 1 {panic!()};
 	let a = &args[0];
-	let Expression::Quantity(q) = a else {panic!()};
+	let Expression::Quantity(l, q) = a else {panic!()};
 
 
 	match f {
-		Function::NoUnit => { return Ok(Expression::Quantity(q.without_unit())); }
-		Function::ToBase => { return Ok(Expression::Quantity(q.convert_to_base())); }
+		Function::NoUnit => { return Ok(Expression::Quantity(*loc + *l, q.without_unit())); }
+		Function::ToBase => { return Ok(Expression::Quantity(*loc + *l, q.convert_to_base())); }
 		_ => {}
 	}
 
 	if !q.unitless() {
-		return Err(EvalError::IncompatibleUnit);
+		return Err((*loc + *l, EvalError::IncompatibleUnit));
 	}
 
 	match f {
-		Function::Abs => { return Ok(Expression::Quantity(q.abs())); },
-		Function::Floor => { return Ok(Expression::Quantity(q.floor())); },
-		Function::Ceil => { return Ok(Expression::Quantity(q.ceil())); },
-		Function::Round => { return Ok(Expression::Quantity(q.round())); },
+		Function::Abs => { return Ok(Expression::Quantity(*loc + *l, q.abs())); },
+		Function::Floor => { return Ok(Expression::Quantity(*loc + *l, q.floor())); },
+		Function::Ceil => { return Ok(Expression::Quantity(*loc + *l, q.ceil())); },
+		Function::Round => { return Ok(Expression::Quantity(*loc + *l, q.round())); },
 
-		Function::NaturalLog => { return Ok(Expression::Quantity(q.ln())); },
-		Function::TenLog => { return Ok(Expression::Quantity(q.log10())); },
+		Function::NaturalLog => { return Ok(Expression::Quantity(*loc + *l, q.ln())); },
+		Function::TenLog => { return Ok(Expression::Quantity(*loc + *l, q.log10())); },
 
-		Function::Sin => { return Ok(Expression::Quantity(q.sin())); },
-		Function::Cos => { return Ok(Expression::Quantity(q.cos())); },
-		Function::Tan => { return Ok(Expression::Quantity(q.tan())); },
-		Function::Asin => { return Ok(Expression::Quantity(q.asin())); },
-		Function::Acos => { return Ok(Expression::Quantity(q.acos())); },
-		Function::Atan => { return Ok(Expression::Quantity(q.atan())); },
+		Function::Sin => { return Ok(Expression::Quantity(*loc + *l, q.sin())); },
+		Function::Cos => { return Ok(Expression::Quantity(*loc + *l, q.cos())); },
+		Function::Tan => { return Ok(Expression::Quantity(*loc + *l, q.tan())); },
+		Function::Asin => { return Ok(Expression::Quantity(*loc + *l, q.asin())); },
+		Function::Acos => { return Ok(Expression::Quantity(*loc + *l, q.acos())); },
+		Function::Atan => { return Ok(Expression::Quantity(*loc + *l, q.atan())); },
 
-		Function::Csc => { return Ok(Expression::Quantity(q.csc())); },
-		Function::Sec => { return Ok(Expression::Quantity(q.sec())); },
-		Function::Cot => { return Ok(Expression::Quantity(q.cot())); },
+		Function::Csc => { return Ok(Expression::Quantity(*loc + *l, q.csc())); },
+		Function::Sec => { return Ok(Expression::Quantity(*loc + *l, q.sec())); },
+		Function::Cot => { return Ok(Expression::Quantity(*loc + *l, q.cot())); },
 
-		Function::Sinh => { return Ok(Expression::Quantity(q.sinh())); },
-		Function::Cosh => { return Ok(Expression::Quantity(q.cosh())); },
-		Function::Tanh => { return Ok(Expression::Quantity(q.tanh())); },
-		Function::Asinh => { return Ok(Expression::Quantity(q.asinh())); },
-		Function::Acosh => { return Ok(Expression::Quantity(q.acosh())); },
-		Function::Atanh => { return Ok(Expression::Quantity(q.atanh())); },
+		Function::Sinh => { return Ok(Expression::Quantity(*loc + *l, q.sinh())); },
+		Function::Cosh => { return Ok(Expression::Quantity(*loc + *l, q.cosh())); },
+		Function::Tanh => { return Ok(Expression::Quantity(*loc + *l, q.tanh())); },
+		Function::Asinh => { return Ok(Expression::Quantity(*loc + *l, q.asinh())); },
+		Function::Acosh => { return Ok(Expression::Quantity(*loc + *l, q.acosh())); },
+		Function::Atanh => { return Ok(Expression::Quantity(*loc + *l, q.atanh())); },
 
-		Function::Csch => { return Ok(Expression::Quantity(q.csch())); },
-		Function::Sech => { return Ok(Expression::Quantity(q.sech())); },
-		Function::Coth => { return Ok(Expression::Quantity(q.coth())); },
+		Function::Csch => { return Ok(Expression::Quantity(*loc + *l, q.csch())); },
+		Function::Sech => { return Ok(Expression::Quantity(*loc + *l, q.sech())); },
+		Function::Coth => { return Ok(Expression::Quantity(*loc + *l, q.coth())); },
 
 		Function::ToBase
 		| Function::NoUnit
-		=> panic!()
+		=> unreachable!()
 	}
 }
\ No newline at end of file
diff --git a/src/evaluate/operator.rs b/src/evaluate/operator.rs
index a21d88a..fff2404 100644
--- a/src/evaluate/operator.rs
+++ b/src/evaluate/operator.rs
@@ -1,12 +1,15 @@
-use std::collections::VecDeque;
-
+use crate::parser::LineLocation;
 use crate::quantity::Quantity;
 use crate::parser::Operator;
 use crate::parser::Expression;
 use super::EvalError;
 use crate::context::Context;
 
-pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut Context) -> Result<Option<Expression>, EvalError> {
+
+pub fn eval_operator(g: &Expression, context: &mut Context) -> Result<Option<Expression>, (LineLocation, EvalError)> {
+
+	let Expression::Operator(op_loc, op, args) = g else {panic!()};
+
 	match op {
 		Operator::Function(_) => unreachable!("Functions are handled seperately."),
 
@@ -14,41 +17,61 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			if args.len() != 2 { panic!() };
 			let b = &args[1];
 
-			if let Expression::Variable(s) = &args[0] {
-				context.push_var(s.clone(), b.clone());
+			if let Expression::Variable(l, s) = &args[0] {
+				let r = context.push_var(s.clone(), b.clone());
+				if r.is_err() { return Err((*l, EvalError::BadDefineName)); }
 				return Ok(Some(b.clone()));
-			} else { return Err(EvalError::BadDefineName); }
+			} else { return Err((args[0].get_linelocation(), EvalError::BadDefineName)); }
 		},
 
 		Operator::Negative => {
 			if args.len() != 1 { panic!() };
 			let args = &args[0];
 
-			if let Expression::Quantity(v) = args {
-				return Ok(Some(Expression::Quantity(-v.clone())));
+			if let Expression::Quantity(l, v) = args {
+				return Ok(Some(Expression::Quantity(*l, -v.clone())));
 			} else { return Ok(None); }
 		},
 
 		Operator::Add => {
 			let mut sum: Quantity;
-			if let Expression::Quantity(s) = &args[0] {
+			let mut loc: LineLocation;
+			if let Expression::Quantity(l, s) = &args[0] {
 				sum = s.clone();
+				loc = l.clone();
 			} else { return Ok(None); };
 
+
+			// Flag that is set to true if we find incompatible units.
+			// We don't stop right away because we need to add all linelocations
+			// to show a pretty error.
+			let mut incompatible_units = false;
+
 			let mut i: usize = 1;
 			while i < args.len() {
 				let j = &args[i];
-				if let Expression::Quantity(v) = j {
+				if let Expression::Quantity(l, v) = j {
 
 					if !sum.unit.compatible_with(&v.unit) {
-						return Err(EvalError::IncompatibleUnit);
+						incompatible_units = true;
 					}
 
-					sum += v.clone();
-				} else { return Ok(None); }
+					if !incompatible_units { sum += v.clone(); }
+					loc += *l;
+				} else {
+					if incompatible_units {
+						return Err((loc + *op_loc, EvalError::IncompatibleUnit));
+					}
+					return Ok(None);
+				}
 				i += 1;
 			}
-			return Ok(Some(Expression::Quantity(sum)));
+
+			if incompatible_units {
+				return Err((loc + *op_loc, EvalError::IncompatibleUnit));
+			}
+
+			return Ok(Some(Expression::Quantity(loc + *op_loc, sum)));
 		},
 
 		Operator::Subtract => {
@@ -56,9 +79,9 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let a = &args[0];
 			let b = &args[1];
 
-			if let Expression::Quantity(a) = a {
-				if let Expression::Quantity(b) = b {
-					return Ok(Some(Expression::Quantity(a.clone() - b.clone())));
+			if let Expression::Quantity(la, a) = a {
+				if let Expression::Quantity(lb, b) = b {
+					return Ok(Some(Expression::Quantity(*la + *lb, a.clone() - b.clone())));
 				}
 			}
 
@@ -72,10 +95,10 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let a = &args[0];
 			let b = &args[1];
 
-			if let Expression::Quantity(a) = a {
-				if let Expression::Quantity(b) = b {
-					if b.is_zero() { return Err(EvalError::ZeroDivision); }
-					return Ok(Some(Expression::Quantity(a.clone() / b.clone())));
+			if let Expression::Quantity(la, a) = a {
+				if let Expression::Quantity(lb, b) = b {
+					if b.is_zero() { return Err((*la + *lb, EvalError::ZeroDivision)); }
+					return Ok(Some(Expression::Quantity(*la + *lb, a.clone() / b.clone())));
 				}
 			}
 
@@ -84,14 +107,23 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 
 		Operator::ImplicitMultiply |
 		Operator::Multiply => {
-			let mut prod = Quantity::new_rational(1f64).unwrap();
-			for i in args.iter() {
-				let j = i;
-				if let Expression::Quantity(v) = j {
+			let mut prod: Quantity;
+			let mut loc: LineLocation;
+			if let Expression::Quantity(l, s) = &args[0] {
+				prod = s.clone();
+				loc = l.clone();
+			} else { return Ok(None); };
+
+			let mut i: usize = 1;
+			while i < args.len() {
+				let j = &args[i];
+				if let Expression::Quantity(l, v) = j {
 					prod *= v.clone();
+					loc += *l;
 				} else { return Ok(None); }
+				i += 1;
 			}
-			return Ok(Some(Expression::Quantity(prod)));
+			return Ok(Some(Expression::Quantity(loc + *op_loc, prod)));
 		},
 
 		Operator::ModuloLong
@@ -100,18 +132,18 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let a = &args[0];
 			let b = &args[1];
 
-			if let Expression::Quantity(va) = a {
-				if let Expression::Quantity(vb) = b {
+			if let Expression::Quantity(la, va) = a {
+				if let Expression::Quantity(lb, vb) = b {
 
 					if !(va.unitless() && vb.unitless()) {
-						return Err(EvalError::IncompatibleUnit);
+						return Err((*la + *lb + *op_loc, EvalError::IncompatibleUnit));
 					}
 
-					if vb <= &Quantity::new_rational(1f64).unwrap() { return Err(EvalError::BadMath); }
-					if va.fract() != Quantity::new_rational(0f64).unwrap() { return Err(EvalError::BadMath); }
-					if vb.fract() != Quantity::new_rational(0f64).unwrap() { return Err(EvalError::BadMath); }
+					if vb <= &Quantity::new_rational(1f64).unwrap() { return Err((*la + *lb + *op_loc, EvalError::BadMath)); }
+					if va.fract() != Quantity::new_rational(0f64).unwrap() { return Err((*la + *lb + *op_loc, EvalError::BadMath)); }
+					if vb.fract() != Quantity::new_rational(0f64).unwrap() { return Err((*la + *lb + *op_loc, EvalError::BadMath)); }
 
-					return Ok(Some(Expression::Quantity(va.clone() % vb.clone())));
+					return Ok(Some(Expression::Quantity(*la + *lb + *op_loc, va.clone() % vb.clone())));
 				} else { return Ok(None); }
 			} else { return Ok(None); }
 		},
@@ -121,13 +153,13 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let a = &args[0];
 			let b = &args[1];
 
-			if let Expression::Quantity(va) = a {
-				if let Expression::Quantity(vb) = b {
+			if let Expression::Quantity(la, va) = a {
+				if let Expression::Quantity(lb, vb) = b {
 					let n = va.clone().convert_to(vb.clone());
 					if n.is_none() {
-						return Err(EvalError::IncompatibleUnit);
+						return Err((*la + *lb + *op_loc, EvalError::IncompatibleUnit));
 					}
-					return Ok(Some(Expression::Quantity(n.unwrap())));
+					return Ok(Some(Expression::Quantity(*la + *lb, n.unwrap())));
 				} else { return Ok(None); }
 			} else { return Ok(None); }
 		},
@@ -137,11 +169,11 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			if args.len() != 1 { panic!() }
 			let a = &args[0];
 
-			if let Expression::Quantity(va) = a {
-				if va.is_negative() { return Err(EvalError::BadMath); }
-				let p = va.pow(Quantity::new_rational_from_string("0.5").unwrap());
-				if p.is_nan() {return Err(EvalError::BadMath);}
-				return Ok(Some(Expression::Quantity(p)));
+			if let Expression::Quantity(l, v) = a {
+				if v.is_negative() { return Err((*l + *op_loc, EvalError::BadMath)); }
+				let p = v.pow(Quantity::new_rational_from_string("0.5").unwrap());
+				if p.is_nan() {return Err((*l + *op_loc, EvalError::BadMath));}
+				return Ok(Some(Expression::Quantity(*l, p)));
 			} else { return Ok(None); }
 		},
 
@@ -150,20 +182,20 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			let a = &args[0];
 			let b = &args[1];
 
-			if let Expression::Quantity(va) = a {
-				if let Expression::Quantity(vb) = b {
+			if let Expression::Quantity(la, va) = a {
+				if let Expression::Quantity(lb, vb) = b {
 
 					if !vb.unitless() {
-						return Err(EvalError::IncompatibleUnit);
+						return Err((*lb, EvalError::IncompatibleUnit));
 					}
 
 					if va.is_zero() && vb.is_negative() {
-						return Err(EvalError::ZeroDivision);
+						return Err((*la + *lb + *op_loc, EvalError::ZeroDivision));
 					}
 
 					let p = va.pow(vb.clone());
-					if p.is_nan() {return Err(EvalError::BadMath);}
-					return Ok(Some(Expression::Quantity(p)));
+					if p.is_nan() {return Err((*la + *lb + *op_loc, EvalError::BadMath));}
+					return Ok(Some(Expression::Quantity(*la + *lb + *op_loc, p)));
 				} else { return Ok(None); }
 			} else { return Ok(None); }
 		},
@@ -172,14 +204,14 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 			if args.len() != 1 {panic!()};
 			let args = &args[0];
 
-			if let Expression::Quantity(v) = args {
+			if let Expression::Quantity(l, v) = args {
 
 				if !v.unitless() {
-					return Err(EvalError::IncompatibleUnit);
+					return Err((*l + *op_loc, EvalError::IncompatibleUnit));
 				}
 
-				if !v.fract().is_zero() { return Err(EvalError::BadMath); }
-				if v > &Quantity::new_rational(50_000f64).unwrap() { return Err(EvalError::TooBig); }
+				if !v.fract().is_zero() { return Err((*l + *op_loc, EvalError::BadMath)); }
+				if v > &Quantity::new_rational(50_000f64).unwrap() { return Err((*l + *op_loc, EvalError::TooBig)); }
 
 				let mut prod = Quantity::new_rational(1f64).unwrap();
 				let mut u = v.clone();
@@ -188,7 +220,7 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Expression>, context: &mut C
 					u = u - Quantity::new_rational(1f64).unwrap();
 				}
 
-				return Ok(Some(Expression::Quantity(prod)));
+				return Ok(Some(Expression::Quantity(*l + *op_loc, prod)));
 			} else { return Ok(None); }
 		}
 	};
diff --git a/src/parser/expression/expression.rs b/src/parser/expression/expression.rs
index 8150356..11a6e21 100644
--- a/src/parser/expression/expression.rs
+++ b/src/parser/expression/expression.rs
@@ -3,24 +3,26 @@ use crate::quantity::Quantity;
 
 use super::Operator;
 use super::Constant;
+use super::super::LineLocation;
+
 
 /// Expressions represent logical objects in an expession.
 #[derive(Debug)]
 #[derive(Clone)]
 pub enum Expression {
-	Variable(String),
-	Quantity(Quantity),
-	Constant(Constant),
-	Operator(Operator, VecDeque<Expression>),
+	Variable(LineLocation, String),
+	Quantity(LineLocation, Quantity),
+	Constant(LineLocation, Constant),
+	Operator(LineLocation, Operator, VecDeque<Expression>),
 }
 
 impl ToString for Expression {
 	fn to_string(&self) -> String {
 		match self {
-			Expression::Quantity(v) => v.to_string(),
-			Expression::Constant(c) => c.to_string(),
-			Expression::Variable(s) => s.clone(),
-			Expression::Operator(o,a) => o.print(a)
+			Expression::Quantity(_, v) => v.to_string(),
+			Expression::Constant(_, c) => c.to_string(),
+			Expression::Variable(_, s) => s.clone(),
+			Expression::Operator(_, o,a) => o.print(a)
 		}
 	}
 }
@@ -30,16 +32,16 @@ impl Expression {
 	// This sometimes leads to different--usually more verbose--behavior.
 	pub fn to_string_outer(&self) -> String {
 		match self {
-			Expression::Quantity(v) => v.to_string_outer(),
-			Expression::Constant(c) => c.to_string(),
-			Expression::Variable(s) => s.clone(),
-			Expression::Operator(o,a) => o.print(a)
+			Expression::Quantity(_, v) => v.to_string_outer(),
+			Expression::Constant(_, c) => c.to_string(),
+			Expression::Variable(_, s) => s.clone(),
+			Expression::Operator(_, o,a) => o.print(a)
 		}
 	}
 
 	pub fn is_quantity(&self) -> bool {
 		match self {
-			Expression::Quantity(_) => true,
+			Expression::Quantity(_,_) => true,
 			_ => false
 		}
 	}
@@ -47,7 +49,7 @@ impl Expression {
 	#[inline(always)]
 	pub fn get_args_mut(&mut self) -> Option<&mut VecDeque<Expression>> {
 		match self {
-			Expression::Operator(_, ref mut a) => Some(a),
+			Expression::Operator(_, _, ref mut a) => Some(a),
 			_ => None
 		}
 	}
@@ -55,7 +57,7 @@ impl Expression {
 	#[inline(always)]
 	pub fn get_args(&self) -> Option<&VecDeque<Expression>> {
 		match self {
-			Expression::Operator(_, ref a) => Some(a),
+			Expression::Operator(_, _, ref a) => Some(a),
 			_ => None
 		}
 	}
@@ -83,4 +85,23 @@ impl Expression {
 		}
 		return Some(g);
 	}
+
+	pub fn get_linelocation(&self) -> LineLocation {
+		match self {
+			Expression::Quantity(l, _)
+			| Expression::Constant(l, _)
+			| Expression::Variable(l, _)
+			| Expression::Operator(l, _,_)
+			=> { l.clone() }
+		}
+	}
+
+	pub fn set_linelocation(&mut self, loc: &LineLocation) {
+		match self {
+			Expression::Quantity(l, _) => { *l = *loc },
+			Expression::Constant(l, _) => { *l = *loc },
+			Expression::Variable(l, _) => { *l = *loc },
+			Expression::Operator(l, _,_) => { *l = *loc },
+		}
+	}
 }
\ No newline at end of file
diff --git a/src/parser/expression/operator.rs b/src/parser/expression/operator.rs
index a03bb4f..e5bffc0 100644
--- a/src/parser/expression/operator.rs
+++ b/src/parser/expression/operator.rs
@@ -110,7 +110,7 @@ impl Operator {
 	#[inline(always)]
 	fn add_parens_to_arg(&self, arg: &Expression) -> String {
 		let mut astr: String = arg.to_string();
-		if let Expression::Operator(o,_) = arg {
+		if let Expression::Operator(_, o,_) = arg {
 			if o < self {
 				astr = format!("({})", astr);
 			}
@@ -121,7 +121,7 @@ impl Operator {
 	#[inline(always)]
 	fn add_parens_to_arg_strict(&self, arg: &Expression) -> String {
 		let mut astr: String = arg.to_string();
-		if let Expression::Operator(o,_) = arg {
+		if let Expression::Operator(_, o,_) = arg {
 			if o <= self {
 				astr = format!("({})", astr);
 			}
@@ -220,15 +220,15 @@ impl Operator {
 				// multiplied by a unit (like 10 m)
 				// Times sign should stay in all other cases.
 				let no_times = {
-					if let Expression::Quantity(p) = a {
-						if let Expression::Quantity(q) = b {
+					if let Expression::Quantity(_, p) = a {
+						if let Expression::Quantity(_, q) = b {
 							p.unitless() && !q.unitless()
 						} else {false}
 					} else {false}
 				};
 
 				if no_times {
-					let Expression::Quantity(u) = b else {panic!()};
+					let Expression::Quantity(_, u) = b else {panic!()};
 					if u.unit.no_space() {
 						return format!("{}{}",
 							self.add_parens_to_arg_strict(a),
@@ -252,7 +252,7 @@ impl Operator {
 				let a = &args[0];
 				let b = &args[1];
 
-				if let Expression::Quantity(q) = a {
+				if let Expression::Quantity(_, q) = a {
 					if q.is_one() {
 						return format!("{}⁻¹",
 							self.add_parens_to_arg_strict(b)
diff --git a/src/parser/linelocation.rs b/src/parser/linelocation.rs
new file mode 100644
index 0000000..c48a203
--- /dev/null
+++ b/src/parser/linelocation.rs
@@ -0,0 +1,59 @@
+use std::cmp;
+use std::ops::Add;
+use std::ops::AddAssign;
+use std::cmp::Ordering;
+
+/// Specifies the location of a token in an input string.
+#[derive(Debug)]
+#[derive(Copy, Clone)]
+pub struct LineLocation {
+	pub pos: usize,
+	pub len: usize
+}
+
+impl LineLocation {
+	pub fn zero(&self) -> bool {
+		return self.pos == 0 && self.len == 0
+	}
+}
+
+impl PartialEq for LineLocation {
+	fn eq(&self, other: &Self) -> bool {
+		self.pos == other.pos &&
+		self.len ==other.len
+	}
+}
+
+impl PartialOrd for LineLocation {
+	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+		return self.pos.partial_cmp(&other.pos);
+	}
+}
+
+impl Add for LineLocation {
+	type Output = Self;
+
+	fn add(self, other: Self) -> Self::Output {
+		if self.zero() { return other; }
+		if other.zero() { return self; }
+
+		let start = cmp::min(self.pos, other.pos);
+		let end = cmp::max(self.pos+self.len-1, other.pos+other.len-1);
+		return LineLocation{
+			pos: start,
+			len: end - start+1
+		};
+	}
+}
+
+impl AddAssign for LineLocation where {
+	fn add_assign(&mut self, other: Self) {
+		if self.zero() {*self = other}
+		if other.zero() { return }
+
+		let start = cmp::min(self.pos, other.pos);
+		let end = cmp::max(self.pos+self.len-1, other.pos+other.len-1);
+		self.pos = start;
+		self.len = end - start + 1;
+	}
+}
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 495c1f5..7717f11 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -3,11 +3,11 @@ mod stage;
 mod token;
 mod parsererror;
 mod expression;
+mod linelocation;
 
 use self::{
 	token::Token,
 	parsererror::ParserError,
-	parsererror::LineLocation
 };
 
 pub use self::{
@@ -15,6 +15,7 @@ pub use self::{
 	expression::Constant,
 	expression::Operator,
 	expression::Function,
+	linelocation::LineLocation,
 };
 
 use crate::context::Context;
@@ -27,7 +28,6 @@ pub fn parse(
 	let expressions = stage::tokenize(s);
 	let (_, expressions) = stage::find_subs(expressions);
 	let g = stage::groupify(expressions)?;
-
 	let g = stage::treeify(g, context)?;
 
 	return Ok(g);
@@ -50,11 +50,12 @@ pub fn substitute(
 
 	let l = s.chars().count();
 	let expressions = stage::tokenize(s);
-	let (subs, _) = stage::find_subs(expressions);
+	let (mut subs, _) = stage::find_subs(expressions);
 	let mut new_c = l - c;
 
-	for r in subs.iter() {
-		// find_subs gives substitutions in reverse order.
+	while subs.len() > 0 {
+		let r = subs.pop_back().unwrap();
+		// Apply substitutions in reverse order
 
 		if { // Don't substitute if our cursor is inside the substitution
 			c >= r.0.pos &&
diff --git a/src/parser/parsererror.rs b/src/parser/parsererror.rs
index 05e6779..15639d2 100644
--- a/src/parser/parsererror.rs
+++ b/src/parser/parsererror.rs
@@ -1,11 +1,3 @@
-/// Specifies the location of a token in an input string.
-#[derive(Debug)]
-#[derive(Copy, Clone)]
-pub struct LineLocation {
-	pub pos: usize,
-	pub len: usize
-}
-
 /// Types of parser errors.
 /// If we cannot parse a string, one of these is returned.
 #[derive(Debug)]
diff --git a/src/parser/stage/find_subs.rs b/src/parser/stage/find_subs.rs
index aceb668..841ad43 100644
--- a/src/parser/stage/find_subs.rs
+++ b/src/parser/stage/find_subs.rs
@@ -9,12 +9,12 @@ use super::super::{
 pub fn find_subs(
 	mut g: VecDeque<Token>,
 ) -> (
-	Vec<(LineLocation, String)>,
+	VecDeque<(LineLocation, String)>,
 	VecDeque<Token>
 ) {
 
 	// Array of replacements
-	let mut r: Vec<(LineLocation, String)> = Vec::with_capacity(8);
+	let mut r: VecDeque<(LineLocation, String)> = VecDeque::with_capacity(8);
 
 	// New token array, with updated locations
 	let mut n: VecDeque<Token> = VecDeque::with_capacity(g.len());
@@ -22,8 +22,7 @@ pub fn find_subs(
 	let mut offset: usize = 0;
 
 	while g.len() > 0 {
-		// Read in reverse. Very important!
-		let mut t = g.pop_back().unwrap();
+		let mut t = g.pop_front().unwrap();
 
 		let target: Option<&str> = match &mut t {
 			Token::Operator(_, s) => {
@@ -85,11 +84,14 @@ pub fn find_subs(
 		} else {
 			let target = target.unwrap();
 			let l = t.get_mut_line_location();
-			r.push((l.clone(), String::from(target)));
-			*l = LineLocation{ pos: l.pos - offset, len: target.chars().count()};
-			offset += l.len - target.chars().count();
+			r.push_back((l.clone(), String::from(target)));
+
+			let old_len = l.len;
+			let new_len = target.chars().count();
+			*l = LineLocation{ pos: l.pos - offset, len: new_len};
+			offset += old_len - new_len;
 		}
-		n.push_front(t);
+		n.push_back(t);
 	}
 
 	return (r, n);
diff --git a/src/parser/stage/groupify.rs b/src/parser/stage/groupify.rs
index 4c8cc5d..930909c 100644
--- a/src/parser/stage/groupify.rs
+++ b/src/parser/stage/groupify.rs
@@ -194,9 +194,12 @@ pub fn groupify(
 	Token,
 	(LineLocation, ParserError)
 > {
+
+	let last_linelocation = g.back().unwrap().get_line_location().clone();
+
 	// Vector of grouping levels
 	let mut levels: Vec<(LineLocation, VecDeque<Token>)> = Vec::with_capacity(8);
-	levels.push((LineLocation{pos: 0, len: 0}, VecDeque::with_capacity(8)));
+	levels.push((LineLocation{pos: 0, len: last_linelocation.pos + last_linelocation.len}, VecDeque::with_capacity(8)));
 
 	// Makes sure parenthesis are matched
 	let mut i_level = 0;
@@ -212,10 +215,7 @@ pub fn groupify(
 			},
 
 			Token::GroupEnd(l) => {
-				let l = LineLocation {
-					pos: l_now.pos,
-					len: l.len + l.pos - l_now.pos
-				};
+				let l = *l_now + l;
 
 				if i_level == 0 { return Err((l, ParserError::ExtraCloseParen)) }
 				if v_now.len() == 0 { return Err((l, ParserError::EmptyGroup)) }
@@ -259,5 +259,5 @@ pub fn groupify(
 	let (_, mut v) = levels.pop().unwrap();
 	lookback(&mut v)?;
 
-	return Ok(Token::Group(LineLocation{pos:0, len:0}, v));
+	return Ok(Token::Group(LineLocation{pos:0, len:last_linelocation.pos + last_linelocation.len}, v));
 }
\ No newline at end of file
diff --git a/src/parser/stage/tokenize.rs b/src/parser/stage/tokenize.rs
index 00e5ce0..470631d 100644
--- a/src/parser/stage/tokenize.rs
+++ b/src/parser/stage/tokenize.rs
@@ -28,7 +28,7 @@ fn push_token(g: &mut VecDeque<Token>, t: Option<Token>, stop_i: usize) {
 
 		Token::Group(_,_)
 		| Token::Container(_)
-		=> panic!()
+		=> unreachable!()
 	};
 
 
diff --git a/src/parser/stage/treeify.rs b/src/parser/stage/treeify.rs
index ce7d63d..6d626c6 100644
--- a/src/parser/stage/treeify.rs
+++ b/src/parser/stage/treeify.rs
@@ -85,11 +85,8 @@ fn treeify_binary(
 		} {
 			return Ok(false);
 		} else {
-			let tl = *this.get_line_location();
-			return Err((
-				LineLocation{pos: tl.pos, len: l.pos - tl.pos + l.len},
-				ParserError::Syntax
-			));
+			let tl = *this.get_line_location() + *l;
+			return Err((tl, ParserError::Syntax));
 		}
 	}
 
@@ -126,22 +123,33 @@ fn treeify_binary(
 		let left_pre = g_inner.remove(i-1).unwrap();
 		let this_pre = g_inner.remove(i-1).unwrap();
 		let right_pre = g_inner.remove(i-1).unwrap();
-		let left: Expression; let right: Expression;
-		if let Token::Group(_, _) = right_pre { right = treeify(right_pre, context)?; } else {right = right_pre.to_expression(context)?;}
-		if let Token::Group(_, _) = left_pre { left = treeify(left_pre, context)?; } else {left = left_pre.to_expression(context)?;}
+		let mut left: Expression; let mut right: Expression;
+		if let Token::Group(l, _) = right_pre {
+			right = treeify(right_pre, context)?;
+			right.set_linelocation(&(right.get_linelocation() + l));
+		} else {
+			right = right_pre.to_expression(context)?;
+		}
 
-		let o = {
-			let Token::Operator(_, s) = this_pre else {panic!()};
+		if let Token::Group(l, _) = left_pre {
+			left = treeify(left_pre, context)?;
+			left.set_linelocation(&(left.get_linelocation() + l));
+		} else {
+			left = left_pre.to_expression(context)?;
+		}
+
+		let (l, o) = {
+			let Token::Operator(l, s) = this_pre else {panic!()};
 			let o = Operator::from_string(&s);
 			if o.is_none() { panic!() }
-			o.unwrap()
+			(l, o.unwrap())
 		};
 
 		let mut new_token_args: VecDeque<Expression> = VecDeque::with_capacity(2);
 		new_token_args.push_back(left);
 		new_token_args.push_back(right);
 
-		g_inner.insert(i-1, Token::Container(Expression::Operator(o, new_token_args)));
+		g_inner.insert(i-1, Token::Container(Expression::Operator(l, o, new_token_args)));
 
 		return Ok(true);
 	} else {
@@ -204,12 +212,8 @@ fn treeify_unary(
 	}
 
 	if let Token::Operator(l, _) = next {
-		let tl = *this.get_line_location();
-		return Err((
-			LineLocation{pos: tl.pos, len: l.pos - tl.pos + l.len},
-			ParserError::Syntax
-		));
-
+		let tl = *this.get_line_location() + *l;
+		return Err((tl, ParserError::Syntax));
 	} else {
 
 		// This operator
@@ -239,28 +243,34 @@ fn treeify_unary(
 
 		if next_op.is_none() || this_op > next_op.unwrap() {
 			let this_pre = g_inner.remove(i).unwrap();
-			let next_pre: Token; let next: Expression;
+			let next_pre: Token; let mut next: Expression;
 			if left_associative {
 				next_pre = g_inner.remove(i-1).unwrap();
 			} else {
 				next_pre = g_inner.remove(i).unwrap();
 			}
-			if let Token::Group(_, _) = next_pre { next = treeify(next_pre, context)?; } else { next = next_pre.to_expression(context)? }
+			if let Token::Group(l, _) = next_pre {
+				next = treeify(next_pre, context)?;
+				next.set_linelocation(&(next.get_linelocation() + l));
+			} else {
+				next = next_pre.to_expression(context)?;
+			}
 
-			let o = {
-				let Token::Operator(_, s) = this_pre else {panic!()};
+
+			let (l, o) = {
+				let Token::Operator(l, s) = this_pre else {panic!()};
 				let o = Operator::from_string(&s);
 				if o.is_none() { panic!() }
-				o.unwrap()
+				(l, o.unwrap())
 			};
 
 			let mut new_token_args: VecDeque<Expression> = VecDeque::with_capacity(3);
 			new_token_args.push_back(next);
 
 			if left_associative {
-				g_inner.insert(i-1, Token::Container(Expression::Operator(o, new_token_args)));
+				g_inner.insert(i-1, Token::Container(Expression::Operator(l, o, new_token_args)));
 			} else {
-				g_inner.insert(i, Token::Container(Expression::Operator(o, new_token_args)));
+				g_inner.insert(i, Token::Container(Expression::Operator(l, o, new_token_args)));
 			}
 
 			return Ok(true);
diff --git a/src/parser/token.rs b/src/parser/token.rs
index eb54a69..f119b03 100644
--- a/src/parser/token.rs
+++ b/src/parser/token.rs
@@ -74,20 +74,20 @@ impl Token {
 					return Err((l, ParserError::BadNumber))
 				}
 
-				return Ok(Expression::Quantity(r.unwrap()));
+				return Ok(Expression::Quantity(l, r.unwrap()));
 			},
 
-			Token::Word(_l, s) => {
+			Token::Word(l, s) => {
 
 				let c = Constant::from_string(&s);
-				if c.is_some() { return Ok(Expression::Constant(c.unwrap())); }
+				if c.is_some() { return Ok(Expression::Constant(l, c.unwrap())); }
 
 				let c = Unit::from_string(&s);
-				if c.is_some() { return Ok(Expression::Quantity(c.unwrap())); }
+				if c.is_some() { return Ok(Expression::Quantity(l, c.unwrap())); }
 
 				let c = context.get_variable(&s);
-				if c.is_some() { return Ok(Expression::Variable(s)); }
-				return Ok(Expression::Variable(s));
+				if c.is_some() { return Ok(Expression::Variable(l, s)); }
+				return Ok(Expression::Variable(l, s));
 			}
 
 			Token::Container(v) => { return Ok(v); }
@@ -96,7 +96,7 @@ impl Token {
 			| Token::GroupStart(_)
 			| Token::GroupEnd(_)
 			| Token::Group(_, _)
-			=> panic!()
+			=> panic!("This token cannot be converted to an expression")
 		};
 	}
 
diff --git a/src/quantity/quantity.rs b/src/quantity/quantity.rs
index 1c2ec2d..d8cd4ab 100644
--- a/src/quantity/quantity.rs
+++ b/src/quantity/quantity.rs
@@ -111,6 +111,37 @@ impl Quantity {
 		return Some(n.mul_no_convert(fa).div_no_convert(fb))
 	}
 
+	pub fn match_units(&mut self, other: &Quantity) {
+
+		let mut new_units = Quantity::new_rational_from_string("1").unwrap();
+		let mut flag;
+
+		// Check every unit in `self`
+		for (us, ps) in self.unit.get_val() {
+			flag = false;
+
+			// Check if `us` matches some unit in `other`
+			for (uo, _) in other.unit.get_val() {
+				// Use generalized compatible_with check to match reciprocal units
+				// (for example, 1Hz * 1 sec.)
+				let f = Unit::from_free(*uo).compatible_with_power(&Unit::from_free(*us));
+				if f.is_none() { continue; }
+				let f = f.unwrap();
+
+				new_units.insert_unit(uo.clone(), ps.clone() * f);
+				flag = true;
+				break;
+			}
+			if !flag {
+				// If no unit in `other` matches `us`, don't convert `us`
+				new_units.insert_unit(us.clone(), ps.clone());
+			}
+		}
+
+		// Convert self to new units
+		*self = self.convert_to(new_units).unwrap();
+	}
+
 	pub fn convert_to_base(&self) -> Quantity { self.convert_to(self.unit.to_base()).unwrap() }
 }
 
@@ -224,7 +255,7 @@ impl Add for Quantity {
 	type Output = Self;
 
 	fn add(self, other: Self) -> Self::Output {
-		if !self.unit.compatible_with(&other.unit) { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!("Tried to add incompatible units") }
 
 		let mut o = other;
 		if self.unit != o.unit {
@@ -240,7 +271,7 @@ impl Add for Quantity {
 
 impl AddAssign for Quantity where {
 	fn add_assign(&mut self, other: Self) {
-		if !self.unit.compatible_with(&other.unit) { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!("Tried to addassign incompatible units") }
 
 		let mut o = other;
 		if self.unit != o.unit {
@@ -255,7 +286,7 @@ impl Sub for Quantity {
 	type Output = Self;
 
 	fn sub(self, other: Self) -> Self::Output {
-		if !self.unit.compatible_with(&other.unit) { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!("Tried to subtract incompatible units") }
 
 		let mut o = other;
 		if self.unit != o.unit {
@@ -271,7 +302,7 @@ impl Sub for Quantity {
 
 impl SubAssign for Quantity where {
 	fn sub_assign(&mut self, other: Self) {
-		if !self.unit.compatible_with(&other.unit) { panic!() }
+		if !self.unit.compatible_with(&other.unit) { panic!("Tried to subassign incompatible units") }
 
 		let mut o = other;
 		if self.unit != o.unit {
@@ -282,24 +313,13 @@ impl SubAssign for Quantity where {
 	}
 }
 
-
 impl Mul for Quantity {
 	type Output = Self;
 
 	fn mul(self, other: Self) -> Self::Output {
 
-
 		let mut o = other;
-		if self.unit != o.unit {
-			if o.unit.compatible_with(&self.unit) {
-				o = o.convert_to(self.clone()).unwrap()
-			} else {
-				let cf = self.unit.common_factor(&o.unit);
-				if let Some(f) = cf {
-					o = o.convert_to(f).unwrap();
-				}
-			}
-		}
+		o.match_units(&self);
 
 		Quantity {
 			scalar: self.scalar * o.scalar,
@@ -311,18 +331,8 @@ impl Mul for Quantity {
 impl MulAssign for Quantity where {
 	fn mul_assign(&mut self, other: Self) {
 
-
 		let mut o = other;
-		if self.unit != o.unit {
-			if o.unit.compatible_with(&self.unit) {
-				o = o.convert_to(self.clone()).unwrap()
-			} else {
-				let cf = self.unit.common_factor(&o.unit);
-				if let Some(f) = cf {
-					o = o.convert_to(f).unwrap();
-				}
-			}
-		}
+		o.match_units(&self);
 
 		self.scalar *= o.scalar;
 		self.unit *= o.unit;
@@ -335,16 +345,7 @@ impl Div for Quantity {
 	fn div(self, other: Self) -> Self::Output {
 
 		let mut o = other;
-		if self.unit != o.unit {
-			if o.unit.compatible_with(&self.unit) {
-				o = o.convert_to(self.clone()).unwrap()
-			} else {
-				let cf = self.unit.common_factor(&o.unit);
-				if let Some(f) = cf {
-					o = o.convert_to(f).unwrap();
-				}
-			}
-		}
+		o.match_units(&self);
 
 		Quantity {
 			scalar: self.scalar / o.scalar,
@@ -357,16 +358,7 @@ impl DivAssign for Quantity where {
 	fn div_assign(&mut self, other: Self) {
 
 		let mut o = other;
-		if self.unit != o.unit {
-			if o.unit.compatible_with(&self.unit) {
-				o = o.convert_to(self.clone()).unwrap()
-			} else {
-				let cf = self.unit.common_factor(&o.unit);
-				if let Some(f) = cf {
-					o = o.convert_to(f).unwrap();
-				}
-			}
-		}
+		o.match_units(&self);
 
 		self.scalar /= o.scalar;
 		self.unit /= o.unit;
@@ -377,8 +369,8 @@ impl Rem<Quantity> for Quantity {
 	type Output = Self;
 
 	fn rem(self, other: Quantity) -> Self::Output {
-		if !self.unit.unitless() { panic!() }
-		if !other.unit.unitless() { panic!() }
+		if !self.unit.unitless() { panic!("Tried to % a quantity with units") }
+		if !other.unit.unitless() { panic!("Tried to % by a quantity with units") }
 
 		Quantity {
 			scalar: self.scalar % other.scalar,
@@ -397,7 +389,7 @@ impl PartialEq for Quantity {
 
 impl PartialOrd for Quantity {
 	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-		if self.unit != other.unit { panic!() }
+		if self.unit != other.unit { panic!("Tried to compare incompatible units") }
 		self.scalar.partial_cmp(&other.scalar)
 	}
 }
\ No newline at end of file
diff --git a/src/quantity/unit/unit.rs b/src/quantity/unit/unit.rs
index 6faa62a..b068504 100644
--- a/src/quantity/unit/unit.rs
+++ b/src/quantity/unit/unit.rs
@@ -52,7 +52,7 @@ impl ToString for Unit {
 						'7' => '⁷',
 						'8' => '⁸',
 						'9' => '⁹',
-						_ => panic!()
+						_ => unreachable!()
 					});
 				}
 				t.push('·');
@@ -90,7 +90,7 @@ impl ToString for Unit {
 						'7' => '⁷',
 						'8' => '⁸',
 						'9' => '⁹',
-						_ => panic!()
+						_ => unreachable!()
 					});
 				}
 				b.push('·');
@@ -154,6 +154,7 @@ impl Unit {
 	}
 
 	// True if base units are the same
+	// compatible <=> can be converted to
 	pub fn compatible_with(&self, other: &Unit) -> bool {
 		let s = self.clone() * self.to_base_factor().unit;
 		let o = other.clone() * other.to_base_factor().unit;
@@ -161,42 +162,27 @@ impl Unit {
 		return o == s;
 	}
 
-	// True if these two units have a common factor
-	pub fn common_factor(&self, other: &Unit) -> Option<Quantity> {
 
-		if self.unitless() || other.unitless() { return None; }
-
-
-		let mut failed = false;
-
-		// What to convert `other` to before multiplying
-		let mut factor = Quantity::new_rational_from_string("1").unwrap();
+	// True if all base units are the same AND there is a constant factor between their powers.
+	// This is a generalization of `compatible_with`. `compatible_with` is true iff
+	// `compatible_with_power` is one.
+	pub fn compatible_with_power(&self, other: &Unit) -> Option<Scalar> {
 		let mut flag;
-		for (us, _) in self.get_val() {
+		let mut pow_factor: Option<Scalar> = None;
+
+		let sbu = self.to_base().unit;
+		let obu = other.to_base().unit;
+
+		for (us, ps) in sbu.get_val() {
 			flag = false;
-			for (uo, po) in other.get_val() {
-				if {
-					us.to_base().unit.compatible_with(&uo.to_base().unit)
-				} {
-					factor.insert_unit(us.clone(), po.clone());
-					flag = true;
-					break;
-				}
-			}
-			if !flag { failed = true }
-		}
+			for (uo, po) in obu.get_val() {
+				if uo.whole == us.whole {
+					if pow_factor.is_none() {
+						pow_factor = Some(po.clone() / ps.clone());
+					} else if let Some(ref f) = pow_factor {
+						if *f != po.clone() / ps.clone() { return None; }
+					}
 
-		if !failed { return Some(factor);}
-
-
-		let mut factor = Quantity::new_rational_from_string("1").unwrap();
-		for (uo, po) in other.get_val() {
-			flag = false;
-			for (us, _) in self.get_val() {
-				if {
-					us.to_base().unit.compatible_with(&uo.to_base().unit)
-				} {
-					factor.insert_unit(us.clone(), po.clone());
 					flag = true;
 					break;
 				}
@@ -204,7 +190,25 @@ impl Unit {
 			if !flag { return None; }
 		}
 
-		return Some(factor);
+		pow_factor = None;
+		for (uo, po) in obu.get_val() {
+			flag = false;
+			for (us, ps) in sbu.get_val() {
+				if uo.whole == us.whole {
+					if pow_factor.is_none() {
+						pow_factor = Some(po.clone() / ps.clone());
+					} else if let Some(ref f) = pow_factor {
+						if *f != po.clone() / ps.clone() { return None; }
+					}
+
+					flag = true;
+					break;
+				}
+			}
+			if !flag { return None; }
+		}
+
+		return pow_factor;
 	}
 
 	pub fn insert(&mut self, u: FreeUnit, p: Scalar) {
@@ -229,6 +233,7 @@ impl Unit {
 		return u;
 	}
 
+	// Returns a unit `u` so that `self * u` contains only base units.
 	pub fn to_base_factor(&self) -> Quantity {
 		let mut q = Quantity::new_rational(1f64).unwrap();
 
@@ -240,6 +245,7 @@ impl Unit {
 		return q;
 	}
 
+	// Returns a unit `u` equivalent to `self` that contains only base units.
 	pub fn to_base(&self) -> Quantity {
 		let mut q = Quantity::new_rational(1f64).unwrap();
 
diff --git a/src/tests.rs b/src/tests.rs
index 0eebedf..19e94f9 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -210,4 +210,7 @@ fn complex_units() {
 	good_expr("62.137 mi/h", "100 km/h to mph");
 	good_expr("20 mi", "10 mph * 2 hours");
 	good_expr("120 m", "1 (m/s) * 2 min");
+	good_expr("120 m", "(2 min) * (1 m/s)");
+	good_expr("180", "1 Hz * 3 min");
+	good_expr("3600", "1 hour * 1 Hz");
 }
\ No newline at end of file