Internal Redesign:

- Combined MPExpression and MPMutableExpression
- Abstracted children of MPExpression into MPExpressionElement protocol
- Abstracted most of MPExpressionLayout and MPFunctionLayout into common superclass MPLayout

MathPad/MPSumFunction.h

@@ -6,6 +6,7 @@
 //  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
 //
 
+@import Foundation;
 #import "MPFunction.h"
 
 @class MPSumFunction, MPExpression;