Internal Redesign:

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

MathPad/MPDocument.m

@@ -59,9 +59,9 @@
 }
 
 #pragma mark Actions
-
 - (IBAction)changeExpression:(id)sender {
-    [self.resultExpressionView.expressionStorage insertString:@"abc" atIndex:6];
+    [self.resultExpressionView.expressionStorage insertElement:@"abc" atLocation:6];
+    self.resultExpressionView.needsDisplay = YES;
 }
 
 @end