Added the MPExpressionTree Classes

MathPad/MPFunction.h

@@ -9,7 +9,6 @@
 @import Foundation;
 #import "MPExpressionElement.h"
 #import "MPParseError.h"
-#import "MPTerm.h"
 
 #define MPFunctionAccessorImplementation(Accessor, variableName) \
 - (void)set##Accessor:(MPExpression *)value \
@@ -50,7 +49,8 @@
 
 #pragma mark Evaluating Functions
 
-- (MPTerm *)parseWithError:(MPParseError *__autoreleasing *)error; // Override
+- (BOOL)validate:(MPParseError *__autoreleasing *)error; // Override
+- (NSDecimalNumber *)evaluate; // Override
 
 #pragma mark Messages
 - (void)didChangeElementsInRangePath:(MPRangePath *)rangePath