diff --git a/MathPad/MPExpression.h b/MathPad/MPExpression.h
index 28e3bd3..977470e 100644
--- a/MathPad/MPExpression.h
+++ b/MathPad/MPExpression.h
@@ -19,6 +19,14 @@ extern NSString *MPDivisionOperator;
 
 - (instancetype)initWithSymbols:(NSArray *)symbols;
 
+#pragma mark Working With the Expression Tree
+
+@property (nonatomic, weak) MPFunction *parent; // Documentation: Do not set, may be nil
+- (void)functionSymbolChanged:(MPFunction *)symbol
+             atLocalIndexPath:(NSIndexPath *)indexPath; // Index path of change in symbol
+
+- (void)fixSymbols;
+
 #pragma mark Primitive Methods
 
 - (NSUInteger)numberOfSymbols;
@@ -81,6 +89,9 @@ extern NSString *MPDivisionOperator;
 - (void)replaceSymbolsInRange:(NSRange)range
                   withSymbols:(NSArray *)symbols;
 
+- (void)beginEditing;
+- (void)endEditing;
+
 @end
 
 @interface MPMutableExpression (MPMutableExpressionExtensionMethods)
diff --git a/MathPad/MPExpression.m b/MathPad/MPExpression.m
index bdcb62d..63f9fd6 100644
--- a/MathPad/MPExpression.m
+++ b/MathPad/MPExpression.m
@@ -21,8 +21,7 @@ NSString *MPDivisionOperator = @"/";
 
 - (NSInteger)lengthOfSymbol:(id)symbol;
 - (void)validateSymbols:(NSArray *)symbols;
-- (NSArray *)repairedSymbols:(NSArray *)symbols; // Merges subsequent strings
-- (void)repairSymbols:(NSMutableArray *)symbols;
+- (NSArray *)fixedSymbols:(NSArray *)symbols;
 - (void)getSplitOffset:(out NSUInteger *)offset
        inSymbolAtIndex:(out NSUInteger *)symbolIndex
       forSplitLocation:(NSUInteger)loc;
@@ -42,13 +41,28 @@ NSString *MPDivisionOperator = @"/";
     [self validateSymbols:symbols];
     self = [super init];
     if (self) {
-        symbols = [self repairedSymbols:symbols];
         _symbols = [[NSArray alloc] initWithArray:symbols
                                         copyItems:YES];
+        [self fixSymbols];
     }
     return self;
 }
 
+#pragma mark Working With the Expression Tree
+
+- (void)functionSymbolChanged:(MPFunction *)symbol atLocalIndexPath:(NSIndexPath *)indexPath
+{
+    NSUInteger index = [_symbols indexOfObject:symbol];
+    if (index != NSNotFound) {
+        [self.parent childChanged:self atLocalIndexPath:[indexPath indexPathByAddingIndex:index]];
+    }
+}
+
+- (void)fixSymbols
+{
+    _symbols = [self fixedSymbols:_symbols];
+}
+
 #pragma mark Primitive Methods
 
 - (NSUInteger)numberOfSymbols
@@ -108,7 +122,6 @@ NSString *MPDivisionOperator = @"/";
     return 1;
 }
 
-// TODO: Deal with subsequent strings
 - (void)validateSymbols:(NSArray *)symbols
 {
     for (id symbol in symbols) {
@@ -121,31 +134,27 @@ NSString *MPDivisionOperator = @"/";
     }
 }
 
-// TODO: Deal with empty strings
-- (NSArray *)repairedSymbols:(NSArray *)symbols
+- (NSArray *)fixedSymbols:(NSArray *)symbols
 {
     NSMutableArray *mutableSymbols = [symbols mutableCopy];
-    [self repairSymbols:mutableSymbols];
-    return [mutableSymbols copy];
-}
-
-- (void)repairSymbols:(NSMutableArray *)symbols
-{
-    for (NSInteger index = 0; index < symbols.count; index++) {
-        id next = index+1 < symbols.count ? symbols[index+1] : nil;
-        id current = symbols[index];
+    for (NSInteger index = 0; index < mutableSymbols.count; index++) {
+        id next = index+1 < mutableSymbols.count ? mutableSymbols[index+1] : nil;
+        id current = mutableSymbols[index];
         if ([current isString]) {
             if ([current length] == 0) {
-                [symbols removeObjectAtIndex:index];
+                [mutableSymbols removeObjectAtIndex:index];
                 index--;
             } else if ([next isString]) {
                 NSString *new = [NSString stringWithFormat:@"%@%@", current, next];
-                [symbols replaceObjectAtIndex:index withObject:new];
-                [symbols removeObjectAtIndex:index+1];
+                [mutableSymbols replaceObjectAtIndex:index withObject:new];
+                [mutableSymbols removeObjectAtIndex:index+1];
                 index--;
             }
+        } else {
+            [(MPFunction *)current setParent:self];
         }
     }
+    return [mutableSymbols copy];
 }
 
 - (void)getSplitOffset:(out NSUInteger *)offset
@@ -396,20 +405,28 @@ NSString *MPDivisionOperator = @"/";
 
 @end
 
-@implementation MPMutableExpression
+@implementation MPMutableExpression {
+    @protected
+    NSInteger _editCount;
+}
 
 - (instancetype)initWithSymbols:(NSArray *)symbols
 {
     [self validateSymbols:symbols];
     self = [super initWithSymbols:nil];
     if (self) {
-        symbols = [self repairedSymbols:symbols];
         _symbols = [[NSMutableArray alloc] initWithArray:symbols
                                                copyItems:YES];
+        [self fixSymbols];
     }
     return self;
 }
 
+- (void)fixSymbols
+{
+    _symbols = [[self fixedSymbols:_symbols] mutableCopy];
+}
+
 - (NSArray *)symbols
 {
     // Return an immutable array:
@@ -451,7 +468,9 @@ NSString *MPDivisionOperator = @"/";
     }
     
     // Revalidate structure and invalidate length
-    [self repairSymbols:(NSMutableArray *)_symbols];
+    if (_editCount == 0) {
+        [self fixSymbols];
+    }
     _length = 0;
 }
 
@@ -481,6 +500,22 @@ NSString *MPDivisionOperator = @"/";
     *insertionIndex = splitSymbolIndex;
 }
 
+- (void)beginEditing
+{
+    _editCount++;
+}
+
+- (void)endEditing
+{
+    if (_editCount == 0) {
+        return;
+    }
+    _editCount--;
+    if (_editCount == 0) {
+        [self fixSymbols];
+    }
+}
+
 @end
 
 @implementation MPMutableExpression (MPMutableExpressionExtensionMethods)
diff --git a/MathPad/MPFunction.h b/MathPad/MPFunction.h
index 8e2bafa..dc3b7b0 100644
--- a/MathPad/MPFunction.h
+++ b/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;
diff --git a/MathPad/MPFunction.m b/MathPad/MPFunction.m
index a1aa526..6c6e2aa 100644
--- a/MathPad/MPFunction.m
+++ b/MathPad/MPFunction.m
@@ -7,6 +7,7 @@
 //
 
 #import "MPFunction.h"
+#import "MPExpression.h"
 
 @implementation MPFunction
 
@@ -20,7 +21,13 @@
     return self;
 }
 
-#pragma mark Children
+#pragma mark Working With the Expression Tree
+
+- (void)childChanged:(MPExpression *)child atLocalIndexPath:(NSIndexPath *)indexPath
+{
+    [self.parent functionSymbolChanged:self
+                      atLocalIndexPath:[indexPath indexPathByAddingIndex:0]];
+}
 
 - (NSUInteger)numberOfChildren
 {
diff --git a/MathPad/NSObject+MPStringTest.h b/MathPad/NSObject+MPStringTest.h
new file mode 100644
index 0000000..73081d5
--- /dev/null
+++ b/MathPad/NSObject+MPStringTest.h
@@ -0,0 +1,15 @@
+//
+//  NSObject+MPStringTest.h
+//  MathPad
+//
+//  Created by Kim Wittenburg on 20.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+@interface NSObject (MPStringTest)
+
+- (BOOL)isString;
+
+@end
diff --git a/MathPad/NSObject+MPStringTest.m b/MathPad/NSObject+MPStringTest.m
new file mode 100644
index 0000000..6a3fe2f
--- /dev/null
+++ b/MathPad/NSObject+MPStringTest.m
@@ -0,0 +1,18 @@
+//
+//  NSObject+MPStringTest.m
+//  MathPad
+//
+//  Created by Kim Wittenburg on 20.04.14.
+//  Copyright (c) 2014 Kim Wittenburg. All rights reserved.
+//
+
+#import "NSObject+MPStringTest.h"
+
+@implementation NSObject (MPStringTest)
+
+- (BOOL)isString
+{
+    return [self isKindOfClass:[NSString class]];
+}
+
+@end