Added some methods to „Working With the Expression Tree“ section

MathPad/MPFunction.h

@@ -35,10 +35,13 @@
 
 @interface MPFunction (MPFunctionExtensionMethods)
 
-#pragma mark Children
+#pragma mark Working With the Expression Tree
 
 // May be overridden for performance improvements
 - (NSArray *)children;
+- (NSUInteger)indexOfChild:(MPExpression *)child;
+
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath;
 
 #pragma mark Evaluating Functions