Added Expression Tree Inspection Methods

Cleaned Code

MathPad/MPFunction.h

@@ -14,7 +14,11 @@
 
 - (instancetype)init;
 
-#pragma mark Children
+#pragma mark Working With the Expression Tree
+
+@property (nonatomic, weak) MPExpression *parent; // Documentation: Do not set
+- (void)childChanged:(MPExpression *)child
+    atLocalIndexPath:(NSIndexPath *)indexPath;
 
 - (NSUInteger)numberOfChildren;
 - (MPExpression *)childAtIndex:(NSUInteger)index;