diff --git a/MathPad/MPExpression.h b/MathPad/MPExpression.h
index 204c1bd..f5c5ee7 100644
--- a/MathPad/MPExpression.h
+++ b/MathPad/MPExpression.h
@@ -17,7 +17,7 @@
 #pragma mark Primitive Methods
 
 - (NSUInteger)numberOfSymbols;
-- (id)symbolAtIndex:(NSUInteger)index; // Either an NSString or a MPFunction
+- (id)symbolAtIndex:(NSUInteger)index; // Either an NSString or a MPFunction (which can be mutated)
 
 - (double)doubleValue; // Evaluates Expression
 
@@ -77,7 +77,7 @@
 
 @interface MPMutableExpression : MPExpression
 
-- (void)replaceSymbolsInRange:(NSRange)range withExpression:(MPExpression *)anExpression;
+- (void)replaceSymbolsInRange:(NSRange)range withSymbols:(NSArray *)symbols;
 
 @end
 
diff --git a/MathPad/MPFunction.h b/MathPad/MPFunction.h
index e1ccc1c..060814a 100644
--- a/MathPad/MPFunction.h
+++ b/MathPad/MPFunction.h
@@ -8,7 +8,7 @@
 
 @class MPFunction, MPExpression;
 
-@interface MPFunction : NSObject
+@interface MPFunction : NSObject <NSCopying, NSCoding>
 
 #pragma mark Creation Methods
 
@@ -17,28 +17,32 @@
 #pragma mark Children
 
 - (NSUInteger)numberOfChildren;
-- (MPExpression *)childAtIndex;
+- (MPExpression *)childAtIndex:(NSUInteger)index;
 - (void)setChild:(MPExpression *)child atIndex:(NSUInteger)index;
 
 #pragma mark Evaluating Functions
 
 - (double)doubleValue;
 
+#pragma mark Working With Functions
+
+- (BOOL)isEqualToFunction:(MPFunction *)aFunction;
+
 @end
 
 @interface MPFunction (MPFunctionExtensionMethods)
 
 #pragma mark Children
 
+// May be overridden for performance improvements
 - (NSArray *)children;
 
-#pragma mark Evaluating Expressions
+#pragma mark Evaluating Functions
 
 - (float)floatValue;
 - (int)intValue;
 - (NSInteger)integerValue;
 - (long long)longLongValue;
-- (NSString *)stringValue;
 
 - (NSString *)description;