Internal Redesign:

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

MathPad/MPRangePath.m

@@ -7,6 +7,7 @@
 //
 
 #import "MPRangePath.h"
+#import "MPExpression.h"
 
 @implementation MPRangePath
 
@@ -143,7 +144,7 @@
 
 - (MPExpression *)subexpressionWithRangePath:(MPRangePath *)aRangePath
 {
-    MPExpression *targetExpression = [self symbolAtIndexPath:[aRangePath.location indexPathByRemovingLastIndex]];
+    MPExpression *targetExpression = [self elementAtIndexPath:[aRangePath.location indexPathByRemovingLastIndex]];
     if (![targetExpression isKindOfClass:[MPExpression class]]) {
         return nil;
     }