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

MathPad/MPExpression.m

@@ -11,6 +11,7 @@
 #import "MPException.h"
 
 #import "NSObject+MPStringTest.h"
+#import "NSIndexPath+MPRemoveFirstIndex.h"
 
 NSString *MPAdditionOperator = @"+";
 NSString *MPSubtractionOperator = @"-";
@@ -214,7 +215,29 @@ NSString *MPDivisionOperator = @"/";
     return [[self alloc] initWithSymbols:symbols];
 }
 
-#pragma mark Working with Expressions
+#pragma mark Working With the Expression Tree
+
+- (NSUInteger)indexOfSymbol:(id)symbol
+{
+    return [_symbols indexOfObject:symbol];
+}
+
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath
+{
+    if (indexPath.length == 0) {
+        return self;
+    }
+    id symbol = [self symbolAtIndex:[indexPath indexAtPosition:0]];
+    if (indexPath.length == 1) {
+        return symbol;
+    }
+    if ([symbol isKindOfClass:[MPFunction class]]) {
+        return [symbol symbolAtIndexPath:[indexPath indexPathByRemovingFirstIndex]];
+    }
+    return nil;
+}
+
+#pragma mark Working With Expressions
 
 - (NSUInteger)length
 {