diff --git a/src/parser/expression/expression.rs b/src/parser/expression/expression.rs
index 564a162..f366e63 100644
--- a/src/parser/expression/expression.rs
+++ b/src/parser/expression/expression.rs
@@ -11,6 +11,23 @@ use super::super::LineLocation;
 #[derive(Debug)]
 #[derive(Clone)]
 pub enum Expression {
+	// Meaning of `LineLocation`:
+	//
+	// For Variables, Constants, Quantities, Tuples:
+	// If this expression was parsed, LineLocation is what part of the prompt was parsed to get this expression
+	// If this expression is the result of a calculation, LineLocaion is the sum of the LineLocations of
+	//	all expressions used to make it. In other words, it points to the part of the prompt that was evaluated
+	//	to get this new expression.
+	//
+	// For Operators:
+	// Linelocation points to the operator's position in the prompt.
+	// If this is a function, it points to the function name.
+	// If this is `+`, `!`, or etc, it points to that character.
+	// Operator arguments are NOT included in this linelocation.
+	//
+	//
+	// All the above rules are implemented when parsing and evaluating expressions.
+
 	Variable(LineLocation, String),
 	Quantity(LineLocation, Quantity),
 	Constant(LineLocation, Constant),
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 56f4eef..fe3a0d5 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -35,6 +35,12 @@ pub fn parse_no_context(s: &String) -> Result<Expression, (LineLocation, DaisyEr
 	parse(&Context::new(), s)
 }
 
+
+
+// Substitiution replaces certain string with pretty unicode characters.
+// When it is enabled, ALL input strings are substituted. Variable and
+// operator tokens use the replaced string value. Make sure both the
+// original and the replaced strings are handled correctly by the parser.
 pub fn substitute(context: &Context, s: &String) -> String {
 	if !context.config.enable_substituion { return s.clone(); }
 	let (_, s) = substitute_cursor(context, s, s.chars().count());
diff --git a/src/parser/stage/find_subs.rs b/src/parser/stage/find_subs.rs
index 865f407..a7b09d4 100644
--- a/src/parser/stage/find_subs.rs
+++ b/src/parser/stage/find_subs.rs
@@ -62,13 +62,13 @@ fn sub_string(s: &str) -> Option<&'static str> {
 }
 
 
-
-
+// Finds substitutions in an array of tokens.
+// Returns new token array and substitution list.
 pub fn find_subs(
 	mut g: VecDeque<Token>,
 ) -> (
-	VecDeque<(LineLocation, String)>,
-	VecDeque<Token>
+	VecDeque<(LineLocation, String)>, // List of substrings to replace (in order)
+	VecDeque<Token> // New token array, with updated strings and linelocations
 ) {
 
 	// Array of replacements
@@ -103,7 +103,7 @@ pub fn find_subs(
 		};
 
 		if target.is_none() {
-			// Even if nothing changed, we need to update token location
+			// Even if nothing changed, we need to update the new token's linelocation
 			let l = t.get_mut_linelocation();
 			*l = LineLocation{pos: l.pos - offset, len: l.len};
 		} else {