diff --git a/README.md b/README.md
index 9617388..e020b90 100644
--- a/README.md
+++ b/README.md
@@ -7,9 +7,7 @@
  - Write a nice README
  - Handle or avoid recursion errors
  - Fix colors
- - Clean up files
  - Print macro content if only a macro is typed
- - Don't expand numbers until you have to
 
 ## Todo:
  - live syntax check
diff --git a/lamb/runner.py b/lamb/runner.py
index 8576409..2109674 100644
--- a/lamb/runner.py
+++ b/lamb/runner.py
@@ -1,3 +1,4 @@
+from tkinter import E
 from prompt_toolkit import PromptSession
 from prompt_toolkit.formatted_text import FormattedText
 from prompt_toolkit import print_formatted_text as printf
@@ -12,9 +13,10 @@ import lamb.utils as utils
 
 class StopReason(enum.Enum):
 	BETA_NORMAL		= ("class:text", "β-normal form")
-	LOOP_DETECTED	= ("class:warn", "loop detected")
-	MAX_EXCEEDED	= ("class:err", "too many reductions")
-	INTERRUPT		= ("class:warn", "user interrupt")
+	LOOP_DETECTED	= ("class:warn", "Loop detected")
+	MAX_EXCEEDED	= ("class:err", "Too many reductions")
+	INTERRUPT		= ("class:warn", "User interrupt")
+	RECURSION		= ("class:err", "Python Recursion Error")
 
 
 class Runner:
@@ -50,7 +52,10 @@ class Runner:
 		# 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 isinstance(e, tokens.macro_expression):
+			e.bind_variables(ban_macro_name = e.label)
+		else:
+			e.bind_variables()
 		return e
 
 
@@ -63,8 +68,14 @@ class Runner:
 		macro_expansions = 0
 
 		stop_reason = StopReason.MAX_EXCEEDED
+
 		while (self.reduction_limit is None) or (i < self.reduction_limit):
-			r = expr.reduce()
+
+			try:
+				r = expr.reduce()
+			except RecursionError:
+				stop_reason = StopReason.RECURSION
+				break
 			expr = r.output
 
 			#print(expr)
@@ -86,19 +97,37 @@ class Runner:
 			else:
 				i += 1
 
-		out_str = str(r.output) # type: ignore
+		if (
+			stop_reason == StopReason.BETA_NORMAL or
+			stop_reason == StopReason.LOOP_DETECTED
+			):
+			out_str = str(r.output) # type: ignore
 
-		printf(FormattedText([
-			("class:result_header", f"\nExit reason: "),
-			stop_reason.value,
+			printf(FormattedText([
+				("class:result_header", f"\nExit reason: "),
+				stop_reason.value,
 
-			("class:result_header", f"\nReduction count: "),
-			("class:text", str(i)),
+				("class:result_header", f"\nMacro expansions: "),
+				("class:text", str(macro_expansions)),
+
+				("class:result_header", f"\nReductions: "),
+				("class:text", str(i)),
 
 
-			("class:result_header", "\n\n    => "),
-			("class:text", out_str),
-		]), style = utils.style)
+				("class:result_header", "\n\n    => "),
+				("class:text", out_str),
+			]), style = utils.style)
+		else:
+			printf(FormattedText([
+				("class:result_header", f"\nExit reason: "),
+				stop_reason.value,
+
+				("class:result_header", f"\nMacro expansions: "),
+				("class:text", str(macro_expansions)),
+
+				("class:result_header", f"\nReductions: "),
+				("class:text", str(i)),
+			]), style = utils.style)
 
 	def save_macro(self, macro: tokens.macro_expression, *, silent = False) -> None:
 		was_rewritten = macro.label in self.macro_table
diff --git a/lamb/tokens.py b/lamb/tokens.py
index f1a9034..8189761 100755
--- a/lamb/tokens.py
+++ b/lamb/tokens.py
@@ -50,7 +50,7 @@ class LambdaToken:
 	def set_runner(self, runner):
 		self.runner = runner
 
-	def bind_variables(self) -> None:
+	def bind_variables(self, *, ban_macro_name: str | None = None) -> None:
 		pass
 
 	def reduce(self) -> ReductionStatus:
@@ -127,12 +127,7 @@ class free_variable(LambdaToken):
 	def __str__(self):
 		return f"{self.label}"
 
-class command:
-	def set_runner(self, runner):
-		pass
-	def bind_variables(self):
-		pass
-
+class command(LambdaToken):
 	@staticmethod
 	def from_parse(result):
 		return command(
@@ -175,6 +170,10 @@ class macro(LambdaToken):
 			raise TypeError("Can only compare macro with macro")
 		return self.name == other.name
 
+	def bind_variables(self, *, ban_macro_name=None) -> None:
+		if self.name == ban_macro_name:
+			raise ReductionError(f"Cannot use macro \"{ban_macro_name}\" here.")
+
 	def reduce(
 			self,
 			*,
@@ -210,7 +209,7 @@ class macro(LambdaToken):
 				was_reduced = True
 			)
 
-class macro_expression:
+class macro_expression(LambdaToken):
 	"""
 	Represents a line that looks like
 		<name> = <expression>
@@ -229,8 +228,9 @@ class macro_expression:
 
 	def set_runner(self, runner):
 		self.expr.set_runner(runner)
-	def bind_variables(self):
-		self.expr.bind_variables()
+
+	def bind_variables(self, *, ban_macro_name: str | None = None):
+		self.expr.bind_variables(ban_macro_name = ban_macro_name)
 
 	def __init__(self, label: str, expr: LambdaToken):
 		self.label = label
@@ -314,7 +314,8 @@ class lambda_func(LambdaToken):
 			placeholder: macro | None = None,
 			val: bound_variable | None = None,
 			*,
-			binding_self: bool = False
+			binding_self: bool = False,
+			ban_macro_name: str | None = None
 		) -> None:
 		"""
 		Go through this function and all the functions inside it,
@@ -370,19 +371,22 @@ class lambda_func(LambdaToken):
 			self.bind_variables(
 				self.input,
 				new_bound_var,
-				binding_self = True
+				binding_self = True,
+				ban_macro_name = ban_macro_name
 			)
 			self.input = new_bound_var
 
 
 		# Bind variables inside this function.
 		if isinstance(self.output, macro) and placeholder is not None:
+			if self.output.name == ban_macro_name:
+				raise ReductionError(f"Cannot use macro \"{ban_macro_name}\" here.")
 			if self.output == placeholder:
 				self.output = val # type: ignore
 		elif isinstance(self.output, lambda_func):
-			self.output.bind_variables(placeholder, val)
+			self.output.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 		elif isinstance(self.output, lambda_apply):
-			self.output.bind_variables(placeholder, val)
+			self.output.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 
 	def reduce(self) -> ReductionStatus:
 
@@ -484,7 +488,9 @@ class lambda_apply(LambdaToken):
 	def bind_variables(
 			self,
 			placeholder: macro | None = None,
-			val: bound_variable | None = None
+			val: bound_variable | None = None,
+			*,
+			ban_macro_name: str | None = None
 		) -> None:
 		"""
 		Does exactly what lambda_func.bind_variables does,
@@ -499,20 +505,24 @@ class lambda_apply(LambdaToken):
 		# If val and placeholder are None,
 		# everything below should still work as expected.
 		if isinstance(self.fn, macro) and placeholder is not None:
+			if self.fn.name == ban_macro_name:
+				raise ReductionError(f"Cannot use macro \"{ban_macro_name}\" here.")
 			if self.fn == placeholder:
 				self.fn = val # type: ignore
 		elif isinstance(self.fn, lambda_func):
-			self.fn.bind_variables(placeholder, val)
+			self.fn.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 		elif isinstance(self.fn, lambda_apply):
-			self.fn.bind_variables(placeholder, val)
+			self.fn.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 
 		if isinstance(self.arg, macro) and placeholder is not None:
+			if self.arg.name == ban_macro_name:
+				raise ReductionError(f"Cannot use macro \"{ban_macro_name}\" here.")
 			if self.arg == placeholder:
 				self.arg = val # type: ignore
 		elif isinstance(self.arg, lambda_func):
-			self.arg.bind_variables(placeholder, val)
+			self.arg.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 		elif isinstance(self.arg, lambda_apply):
-			self.arg.bind_variables(placeholder, val)
+			self.arg.bind_variables(placeholder, val, ban_macro_name = ban_macro_name)
 
 	def sub_bound_var(
 		self,