diff --git a/.vscode/settings.json b/.vscode/settings.json
index 6a0417c..af7b709 100755
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -7,6 +7,7 @@
         "Packrat",
         "pyparsing",
         "runstatus",
+        "srange",
         "subvar"
     ],
     "python.analysis.typeCheckingMode": "basic"
diff --git a/README.md b/README.md
index 11bfd98..c68e413 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,6 @@
  - Command and macro autocomplete
  - step-by-step reduction
  - Maybe a better icon?
- - Warn when overwriting macro
  - Syntax highlighting: parenthesis, bound variables, macros, etc
  - Pin header to top of screen
  - PyPi package
diff --git a/lamb/__main__.py b/lamb/__main__.py
index 7c12200..43cb809 100755
--- a/lamb/__main__.py
+++ b/lamb/__main__.py
@@ -52,13 +52,11 @@ r.run_lines([
 	"AND = λab.(a F b)",
 	"OR = λab.(a T b)",
 	"XOR = λab.(a (NOT a b) b)",
-	"w = λx.(x x)",
-	"W = w w",
+	"M = λx.(x x)",
+	"W = M M",
 	"Y = λf.( (λx.(f (x x))) (λx.(f (x x))) )",
 	"PAIR = λabi.( i a b )",
-	"inc = λnfa.(f (n f a))",
-	"zero = λax.x",
-	"one = λfx.(f x)"
+	"S = λnfa.(f (n f a))",
 ])
 
 
@@ -89,7 +87,7 @@ while True:
 			("class:err", " "*(e.loc + l) + "^\n"),
 			("class:err", f"Syntax error at char {e.loc}."),
 			("class:text", "\n")
-		]))
+		]), style = utils.style)
 		continue
 	except tokens.ReductionError as e:
 		printf(FormattedText([
diff --git a/lamb/parser.py b/lamb/parser.py
index b0a49d4..1442a1c 100755
--- a/lamb/parser.py
+++ b/lamb/parser.py
@@ -1,82 +1,96 @@
 import pyparsing as pp
 
-# Packrat gives a MAD speed boost.
+# Packrat gives a significant speed boost.
 pp.ParserElement.enablePackrat()
 
-import lamb.tokens as tokens
-import lamb.utils as utils
+class LambdaParser:
+	def make_parser(self):
+		self.lp = pp.Suppress("(")
+		self.rp = pp.Suppress(")")
+		self.pp_expr = pp.Forward()
 
-class Parser:
-	lp = pp.Suppress("(")
-	rp = pp.Suppress(")")
+		# Bound variables are ALWAYS lowercase and single-character.
+		# We still create macro objects from them, they are turned into
+		# bound variables after the expression is built.
+		self.pp_macro = pp.Word(pp.alphas + "_")
+		self.pp_bound = pp.Char(pp.srange("[a-z]"))
+		self.pp_name = self.pp_bound ^ self.pp_macro
+		self.pp_church = pp.Word(pp.nums)
 
-	# Simple tokens
-	pp_expr = pp.Forward()
-	pp_macro = pp.Word(pp.alphas + "_")
-	pp_macro.set_parse_action(tokens.macro.from_parse)
+		# Function calls.
+		#
+		# <exp> <exp>
+		# <exp> <exp> <exp>
+		self.pp_call = pp.Forward()
+		self.pp_call <<= (self.pp_expr | self.pp_bound)[2, ...]
 
-	pp_church = pp.Word(pp.nums)
-	pp_church.set_parse_action(utils.autochurch)
+		# Function definitions, right associative.
+		# Function args MUST be lowercase.
+		#
+		# <var> => <exp>
+		self.pp_lambda_fun = (
+			(pp.Suppress("λ") | pp.Suppress("\\")) +
+			pp.Group(self.pp_bound[1, ...]) +
+			pp.Suppress(".") +
+			(self.pp_expr ^ self.pp_call)
+		)
 
-	# Function calls.
-	# `tokens.lambda_apply.from_parse` handles chained calls.
-	#
-	# <exp> <exp>
-	# <exp> <exp> <exp>
-	pp_call = pp.Forward()
-	pp_call <<= pp_expr[2, ...]
-	pp_call.set_parse_action(tokens.lambda_apply.from_parse)
+		# Assignment.
+		# Can only be found at the start of a line.
+		#
+		# <name> = <exp>
+		self.pp_macro_def = (
+			pp.line_start() +
+			self.pp_macro +
+			pp.Suppress("=") +
+			(self.pp_expr ^ self.pp_call)
+		)
 
-	# Function definitions.
-	# Right associative.
-	#
-	# <var> => <exp>
-	pp_lambda_fun = (
-		(pp.Suppress("λ") | pp.Suppress("\\")) +
-		pp.Group(pp.Char(pp.alphas)[1, ...]) +
-		pp.Suppress(".") +
-		(pp_expr ^ pp_call)
-	)
-	pp_lambda_fun.set_parse_action(tokens.lambda_func.from_parse)
+		self.pp_expr <<= (
+			self.pp_church ^
+			self.pp_lambda_fun ^
+			self.pp_name ^
+			(self.lp + self.pp_expr + self.rp) ^
+			(self.lp + self.pp_call + self.rp)
+		)
 
-	# Assignment.
-	# Can only be found at the start of a line.
-	#
-	# <name> = <exp>
-	pp_macro_def = (
-		pp.line_start() +
-		pp_macro +
-		pp.Suppress("=") +
-		(pp_expr ^ pp_call)
-	)
-	pp_macro_def.set_parse_action(tokens.macro_expression.from_parse)
-
-	pp_expr <<= (
-		pp_church ^
-		pp_lambda_fun ^
-		pp_macro ^
-		(lp + pp_expr + rp) ^
-		(lp + pp_call + rp)
-	)
-
-	pp_command = pp.Suppress(":") + pp.Word(pp.alphas + "_") + pp.Word(pp.alphas + "_")[0, ...]
-	pp_command.set_parse_action(tokens.command.from_parse)
+		self.pp_command = pp.Suppress(":") + pp.Word(pp.alphas + "_") + pp.Word(pp.alphas + "_")[0, ...]
 
 
-	pp_all = (
-		pp_expr ^
-		pp_macro_def ^
-		pp_command ^
-		pp_call
-	)
+		self.pp_all = (
+			self.pp_expr ^
+			self.pp_macro_def ^
+			self.pp_command ^
+			self.pp_call
+		)
 
-	@staticmethod
-	def parse_line(line):
-		return Parser.pp_all.parse_string(
+	def __init__(
+			self,
+			*,
+			action_command,
+			action_macro_def,
+			action_church,
+			action_func,
+			action_bound,
+			action_macro,
+			action_apply
+		):
+
+		self.make_parser()
+
+		self.pp_command.set_parse_action(action_command)
+		self.pp_macro_def.set_parse_action(action_macro_def)
+		self.pp_church.set_parse_action(action_church)
+		self.pp_lambda_fun.set_parse_action(action_func)
+		self.pp_macro.set_parse_action(action_macro)
+		self.pp_bound.set_parse_action(action_bound)
+		self.pp_call.set_parse_action(action_apply)
+
+	def parse_line(self, line: str):
+		return self.pp_all.parse_string(
 			line,
 			parse_all = True
 		)[0]
 
-	@staticmethod
-	def run_tests(lines):
-		return Parser.pp_all.run_tests(lines)
\ No newline at end of file
+	def run_tests(self, lines: list[str]):
+		return self.pp_all.run_tests(lines)
\ No newline at end of file
diff --git a/lamb/runner.py b/lamb/runner.py
index 0da75b7..241f6f5 100644
--- a/lamb/runner.py
+++ b/lamb/runner.py
@@ -1,8 +1,9 @@
 from prompt_toolkit import PromptSession
 
 import lamb.commands as commands
-from lamb.parser import Parser
+from lamb.parser import LambdaParser
 import lamb.tokens as tokens
+import lamb.utils as utils
 import lamb.runstatus as rs
 
 
@@ -11,11 +12,25 @@ class Runner:
 		self.macro_table = {}
 		self.prompt_session = prompt_session
 		self.prompt_message = prompt_message
+		self.parser = LambdaParser(
+			action_command		= tokens.command.from_parse,
+			action_macro_def	= tokens.macro_expression.from_parse,
+			action_church		= utils.autochurch(self),
+			action_func			= tokens.lambda_func.from_parse,
+			action_bound		= tokens.macro.from_parse,
+			action_macro		= tokens.macro.from_parse,
+			action_apply		= tokens.lambda_apply.from_parse
+		)
 
 		# Maximum amount of reductions.
 		# If None, no maximum is enforced.
 		self.reduction_limit: int | None = 300
 
+		# Ensure bound variables are unique.
+		# This is automatically incremented whenever we make
+		# a bound variable.
+		self.bound_variable_counter = 0
+
 	def prompt(self):
 		return self.prompt_session.prompt(message = self.prompt_message)
 
@@ -29,7 +44,8 @@ class Runner:
 		macro_expansions = 0
 
 		while (self.reduction_limit is None) or (i < self.reduction_limit):
-			r = expr.reduce(self.macro_table)
+			print(repr(expr))
+			r = expr.reduce()
 			expr = r.output
 
 			# If we can't reduce this expression anymore,
@@ -55,13 +71,15 @@ class Runner:
 
 	# Apply a list of definitions
 	def run(self, line: str, *, macro_only = False) -> rs.RunStatus:
-		e = Parser.parse_line(line)
+		e = self.parser.parse_line(line)
+		# Give the elements of this expression access to the runner.
+		# Runner must be set BEFORE variables are bound.
+		e.set_runner(self)
+		e.bind_variables()
 
 		# If this line is a macro definition, save the macro.
 		if isinstance(e, tokens.macro_expression):
 			was_rewritten = e.label in self.macro_table
-
-			e.exp.bind_variables()
 			self.macro_table[e.label] = e.exp
 
 			return rs.MacroStatus(
@@ -80,7 +98,6 @@ class Runner:
 
 		# If this line is a plain expression, reduce it.
 		elif isinstance(e, tokens.LambdaToken):
-			e.bind_variables()
 			return self.reduce_expression(e)
 
 		# We shouldn't ever get here.
diff --git a/lamb/tokens.py b/lamb/tokens.py
index 05dadcb..617a577 100755
--- a/lamb/tokens.py
+++ b/lamb/tokens.py
@@ -40,16 +40,18 @@ class ReductionStatus:
 		# this will be false.
 		self.was_reduced = was_reduced
 
-
 class LambdaToken:
 	"""
 	Base class for all lambda tokens.
 	"""
 
+	def set_runner(self, runner):
+		self.runner = runner
+
 	def bind_variables(self) -> None:
 		pass
 
-	def reduce(self, macro_table) -> ReductionStatus:
+	def reduce(self) -> ReductionStatus:
 		return ReductionStatus(
 			was_reduced = False,
 			output = self
@@ -121,7 +123,6 @@ class macro(LambdaToken):
 
 	def reduce(
 			self,
-			macro_table = {},
 			*,
 			# To keep output readable, we avoid expanding macros as often as possible.
 			# Macros are irreducible if force_substitute is false.
@@ -132,10 +133,10 @@ class macro(LambdaToken):
 			auto_free_vars = True
 		) -> ReductionStatus:
 
-		if (self.name in macro_table) and force_substitute:
+		if (self.name in self.runner.macro_table) and force_substitute:
 			if force_substitute: # Only expand macros if we NEED to
 				return ReductionStatus(
-					output = macro_table[self.name],
+					output = self.runner.macro_table[self.name],
 					reduction_type = ReductionType.MACRO_EXPAND,
 					was_reduced = True
 				)
@@ -173,6 +174,11 @@ class macro_expression:
 			result[1]
 		)
 
+	def set_runner(self, runner):
+		self.exp.set_runner(runner)
+	def bind_variables(self):
+		self.exp.bind_variables()
+
 	def __init__(self, label: str, exp: LambdaToken):
 		self.label = label
 		self.exp = exp
@@ -184,14 +190,14 @@ class macro_expression:
 		return f"{self.label} := {self.exp}"
 
 
-bound_variable_counter = 0
 class bound_variable(LambdaToken):
-	def __init__(self, forced_id = None):
-		global bound_variable_counter
+	def __init__(self, name: str, *, runner, forced_id = None):
+		self.original_name = name
+		self.runner = runner
 
 		if forced_id is None:
-			self.identifier = bound_variable_counter
-			bound_variable_counter += 1
+			self.identifier = self.runner.bound_variable_counter
+			self.runner.bound_variable_counter += 1
 		else:
 			self.identifier = forced_id
 
@@ -201,7 +207,7 @@ class bound_variable(LambdaToken):
 		return self.identifier == other.identifier
 
 	def __repr__(self):
-		return f"<in {self.identifier}>"
+		return f"<{self.original_name} {self.identifier}>"
 
 class lambda_func(LambdaToken):
 	"""
@@ -219,15 +225,20 @@ class lambda_func(LambdaToken):
 	def from_parse(result):
 		if len(result[0]) == 1:
 			return lambda_func(
-				macro(result[0][0]),
+				result[0][0],
 				result[1]
 			)
 		else:
 			return lambda_func(
-				macro(result[0].pop(0)),
+				result[0].pop(0),
 				lambda_func.from_parse(result)
 			)
 
+	def set_runner(self, runner):
+		self.runner = runner
+		self.input.set_runner(runner)
+		self.output.set_runner(runner)
+
 	def __init__(
 			self,
 			input_var: macro | bound_variable,
@@ -280,10 +291,14 @@ class lambda_func(LambdaToken):
 		# We only need to check for collisions if we're
 		# binding another function's variable. If this
 		# function starts the bind chain, skip that step.
-		if not ((placeholder is None) and (val is None)):
+		if placeholder is not None:
 			if not binding_self and isinstance(self.input, macro):
 				if self.input == placeholder:
-					raise ReductionError(f"Variable name conflict: \"{self.input.name}\"")
+					raise ReductionError(f"Bound variable name conflict: \"{self.input.name}\"")
+
+				if self.input.name in self.runner.macro_table:
+					raise ReductionError(f"Bound variable name conflict: \"{self.input.name}\" is a macro")
+
 
 		# If this function's variables haven't been bound yet,
 		# bind them BEFORE binding the outer function's.
@@ -292,7 +307,10 @@ class lambda_func(LambdaToken):
 		# functions' variables, we won't be able to detect
 		# name conflicts.
 		if isinstance(self.input, macro) and not binding_self:
-			new_bound_var = bound_variable()
+			new_bound_var = bound_variable(
+				self.input.name,
+				runner = self.runner
+			)
 			self.bind_variables(
 				self.input,
 				new_bound_var,
@@ -310,15 +328,15 @@ class lambda_func(LambdaToken):
 		elif isinstance(self.output, lambda_apply):
 			self.output.bind_variables(placeholder, val)
 
-	def reduce(self, macro_table = {}) -> ReductionStatus:
+	def reduce(self) -> ReductionStatus:
 
-		r = self.output.reduce(macro_table)
+		r = self.output.reduce()
 
 		# If a macro becomes a free variable,
 		# reduce twice.
 		if r.reduction_type == ReductionType.MACRO_TO_FREE:
 			self.output = r.output
-			return self.reduce(macro_table)
+			return self.reduce()
 
 		return ReductionStatus(
 			was_reduced = r.was_reduced,
@@ -394,6 +412,11 @@ class lambda_apply(LambdaToken):
 				)] + result[2:]
 			)
 
+	def set_runner(self, runner):
+		self.runner = runner
+		self.fn.set_runner(runner)
+		self.arg.set_runner(runner)
+
 	def __init__(
 			self,
 			fn: LambdaToken,
@@ -416,9 +439,6 @@ class lambda_apply(LambdaToken):
 		"""
 		Does exactly what lambda_func.bind_variables does,
 		but acts on applications instead.
-
-		There will be little documentation in this method,
-		see lambda_func.bind_variables.
 		"""
 
 		if (placeholder is None) and (val != placeholder):
@@ -474,7 +494,7 @@ class lambda_apply(LambdaToken):
 			new_arg
 		)
 
-	def reduce(self, macro_table = {}) -> ReductionStatus:
+	def reduce(self) -> ReductionStatus:
 
 		# If we can directly apply self.fn, do so.
 		if isinstance(self.fn, lambda_func):
@@ -491,17 +511,16 @@ class lambda_apply(LambdaToken):
 				# Macros must be reduced before we apply them as functions.
 				# This is the only place we force substitution.
 				r = self.fn.reduce(
-					macro_table,
 					force_substitute = True
 				)
 			else:
-				r = self.fn.reduce(macro_table)
+				r = self.fn.reduce()
 
 			# If a macro becomes a free variable,
 			# reduce twice.
 			if r.reduction_type == ReductionType.MACRO_TO_FREE:
 				self.fn = r.output
-				return self.reduce(macro_table)
+				return self.reduce()
 
 			if r.was_reduced:
 				return ReductionStatus(
@@ -514,11 +533,11 @@ class lambda_apply(LambdaToken):
 				)
 
 			else:
-				r = self.arg.reduce(macro_table)
+				r = self.arg.reduce()
 
 				if r.reduction_type == ReductionType.MACRO_TO_FREE:
 					self.arg = r.output
-					return self.reduce(macro_table)
+					return self.reduce()
 
 				return ReductionStatus(
 					was_reduced = r.was_reduced,
diff --git a/lamb/utils.py b/lamb/utils.py
index 23fb1b9..76e31e1 100644
--- a/lamb/utils.py
+++ b/lamb/utils.py
@@ -6,31 +6,33 @@ from importlib.metadata import version
 import lamb.tokens as tokens
 
 
-def autochurch(results):
+def autochurch(runner):
 	"""
 	Makes a church numeral from an integer.
 	"""
 
-	num = int(results[0])
+	def inner(results):
+		num = int(results[0])
 
-	f = tokens.bound_variable()
-	a = tokens.bound_variable()
+		f = tokens.bound_variable("f", runner = runner)
+		a = tokens.bound_variable("a", runner = runner)
 
-	chain = a
+		chain = a
 
-	for i in range(num):
-		chain = tokens.lambda_apply(f, chain)
+		for i in range(num):
+			chain = tokens.lambda_apply(f, chain)
 
-	return tokens.lambda_func(
-		f,
-		tokens.lambda_func(
-			a,
-			chain
+		return tokens.lambda_func(
+			f,
+			tokens.lambda_func(
+				a,
+				chain
+			)
 		)
-	)
+	return inner
 
 
-style = Style.from_dict({
+style = Style.from_dict({ # type: ignore
 	# Basic formatting
 	"text": "#FFFFFF",
 	"warn": "#FFFF00",