diff --git a/MathPad/Base.lproj/MPDocument.xib b/MathPad/Base.lproj/MPDocument.xib
index 9783c54..1600811 100644
--- a/MathPad/Base.lproj/MPDocument.xib
+++ b/MathPad/Base.lproj/MPDocument.xib
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="5056" systemVersion="13C64" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES">
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="5056" systemVersion="13C1021" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES">
     <dependencies>
         <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="5056"/>
     </dependencies>
@@ -31,6 +31,17 @@
                         <rect key="frame" x="20" y="20" width="467" height="96"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
                     </customView>
+                    <button verticalHuggingPriority="750" fixedFrame="YES" translatesAutoresizingMaskIntoConstraints="NO" id="IMg-L0-qdu">
+                        <rect key="frame" x="405" y="-7" width="88" height="32"/>
+                        <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMinY="YES"/>
+                        <buttonCell key="cell" type="push" title="Change" bezelStyle="rounded" alignment="center" borderStyle="border" imageScaling="proportionallyDown" inset="2" id="Xxz-j2-fsI">
+                            <behavior key="behavior" pushIn="YES" lightByBackground="YES" lightByGray="YES"/>
+                            <font key="font" metaFont="system"/>
+                        </buttonCell>
+                        <connections>
+                            <action selector="changeExpression:" target="-2" id="k8U-3Y-8Ch"/>
+                        </connections>
+                    </button>
                 </subviews>
                 <constraints>
                     <constraint firstItem="lcd-Ip-jjR" firstAttribute="top" secondItem="fqc-IQ-ceJ" secondAttribute="bottom" constant="8" symbolic="YES" id="GTC-QI-4wc"/>
diff --git a/MathPad/MPDocument.h b/MathPad/MPDocument.h
index d510304..927eff2 100644
--- a/MathPad/MPDocument.h
+++ b/MathPad/MPDocument.h
@@ -13,4 +13,6 @@
 @property (weak) IBOutlet MPExpressionView *termExpressionView;
 @property (weak) IBOutlet MPExpressionView *resultExpressionView;
 
+- (IBAction)changeExpression:(id)sender;
+
 @end
diff --git a/MathPad/MPDocument.m b/MathPad/MPDocument.m
index c3acc12..6e86a3c 100644
--- a/MathPad/MPDocument.m
+++ b/MathPad/MPDocument.m
@@ -58,4 +58,10 @@
     return YES;
 }
 
+#pragma mark Actions
+
+- (IBAction)changeExpression:(id)sender {
+    [self.resultExpressionView.expressionStorage insertString:@"abc" atIndex:6];
+}
+
 @end
diff --git a/MathPad/MPExpressionLayout.h b/MathPad/MPExpressionLayout.h
index 4fec051..7c05565 100644
--- a/MathPad/MPExpressionLayout.h
+++ b/MathPad/MPExpressionLayout.h
@@ -8,7 +8,7 @@
 
 #import <Foundation/Foundation.h>
 
-@class MPExpressionLayout, MPFunctionLayout, MPExpressionStorage, MPExpression;
+@class MPExpressionLayout, MPFunctionLayout, MPExpressionStorage, MPExpressionView, MPExpression;
 
 @interface MPExpressionLayout : NSObject {
     BOOL _valid;
@@ -30,6 +30,8 @@
 @property (readonly, nonatomic, weak) MPExpressionStorage *expressionStorage;
 @property (readonly, nonatomic, strong) NSIndexPath *expressionPath;
 
+@property (nonatomic, weak) MPExpressionView *expressionView;
+
 - (MPExpression *)expression; // Convenience
 - (NSLayoutManager *)layoutManager;
 - (NSTextContainer *)textContainer;
@@ -38,7 +40,7 @@
 #pragma mark Cache Methods
 
 - (void)invalidate;
-- (void)expressionEditedInRange:(NSRange)range
+- (void)editedExpressionInRange:(NSRange)range
               replacementLength:(NSUInteger)length;
 
 - (BOOL)hasCacheForSymbolAtIndex:(NSUInteger)index;
diff --git a/MathPad/MPExpressionLayout.m b/MathPad/MPExpressionLayout.m
index f5c9c09..6b4da0a 100644
--- a/MathPad/MPExpressionLayout.m
+++ b/MathPad/MPExpressionLayout.m
@@ -10,6 +10,7 @@
 #import "MPExpressionStorage.h"
 #import "MPFunctionLayout.h"
 #import "MPModel.h"
+#import "MPExpressionView.h"
 
 @implementation MPExpressionLayout
 
@@ -71,15 +72,20 @@
 
 #pragma mark Cache Methods
 
+// TODO: Return nil from caching with illegal index
+
 - (void)invalidate
 {
     _valid = NO;
     [self.parent invalidate];
+    [self.expressionView setNeedsDisplay:YES];
 }
 
-- (void)expressionEditedInRange:(NSRange)range replacementLength:(NSUInteger)length
+- (void)editedExpressionInRange:(NSRange)range replacementLength:(NSUInteger)length
 {
-    while (_symbolCache.count < self.expression.numberOfSymbols) {
+    // TODO: New symbols may also be inserted in the middle or at the beginning
+    NSInteger changeInLength = length - range.length;
+    while (_symbolCache.count < (self.expression.numberOfSymbols + changeInLength)) {
         [_symbolCache addObject:[NSNull null]];
     }
     NSMutableArray *newPlaceholders = [[NSMutableArray alloc] initWithCapacity:length];
@@ -101,7 +107,11 @@
 - (MPFunctionLayout *)functionLayoutForFunctionAtIndex:(NSUInteger)index;
 {
     if ([self hasCacheForSymbolAtIndex:index]) {
-        return _symbolCache[index];
+        id cacheObject = _symbolCache[index];
+        if ([cacheObject isKindOfClass:[NSValue class]]) {
+            return nil;
+        }
+        return cacheObject;
     }
      MPFunctionLayout *layout = [MPFunctionLayout functionLayoutForFunctionAtIndexPath:[self.expressionPath indexPathByAddingIndex:index] parent:self];
     while (index >= _symbolCache.count) {
@@ -177,6 +187,8 @@
                   atPoint:(NSPoint)point
 {
     id symbol = [self.expression symbolAtIndex:index];
+    // point.x = point.y = 0;
+    NSLog(@"draw Symbol: %@ at Point: (x: %f, y: %f)", symbol, point.x, point.y);
     if ([symbol isString]) {
         [self.textStorage setString:symbol];
         NSRange glyphRange = [self.layoutManager glyphRangeForTextContainer:self.textContainer];
@@ -184,6 +196,7 @@
                                             atPoint:point];
     } else {
         MPFunctionLayout *layout = [self functionLayoutForFunctionAtIndex:index];
+        NSLog(@"layout: %@, index: %ld", layout, index);
         [layout drawFunctionAtPoint:point];
     }
 }
diff --git a/MathPad/MPExpressionStorage.m b/MathPad/MPExpressionStorage.m
index 7254755..ae665e8 100644
--- a/MathPad/MPExpressionStorage.m
+++ b/MathPad/MPExpressionStorage.m
@@ -8,6 +8,8 @@
 
 #import "MPExpressionStorage.h"
 #import "MPExpressionLayout.h"
+#import "MPFunctionLayout.h"
+#import "MPRangePath.h"
 
 @interface MPExpressionStorage ()
 @property (nonatomic, strong) NSLayoutManager *layoutManager;
@@ -57,4 +59,24 @@
     }
 }
 
+- (void)symbolsChangedInRangePath:(MPRangePath *)rangePath replacementLength:(NSUInteger)length
+{
+    if (rangePath.location.length == 0) {
+        return;
+    }
+    id current = self.expressionLayout;
+    for (NSUInteger position = 1; position < rangePath.location.length-1; position++) {
+        if ([current isKindOfClass:[MPExpressionLayout class]]) {
+            current = [(MPExpressionLayout *)current functionLayoutForFunctionAtIndex:position];
+        } else {
+            current = [(MPFunctionLayout *)current expressionLayoutForChildAtIndex:position];
+        }
+    }
+    if ([current isKindOfClass:[MPExpressionLayout class]]) {
+        [(MPExpressionLayout *)current editedExpressionInRange:rangePath.rangeAtLastIndex replacementLength:length];
+    } else {
+        [(MPFunctionLayout *)current editedChildAtIndex:[rangePath.location indexAtPosition:rangePath.location.length-1]];
+    }
+}
+
 @end
diff --git a/MathPad/MPExpressionView.h b/MathPad/MPExpressionView.h
index b1c1c78..1cc8b70 100644
--- a/MathPad/MPExpressionView.h
+++ b/MathPad/MPExpressionView.h
@@ -20,7 +20,7 @@
 
 #pragma mark Properties
 
-@property (readonly, nonatomic, copy) MPExpressionStorage *expressionStorage;
+@property (readonly, nonatomic, strong) MPExpressionStorage *expressionStorage;
 - (MPExpressionLayout *)expressionLayout; // Convenience Method
 
 @property (nonatomic, getter = isEditable) BOOL editable;
diff --git a/MathPad/MPExpressionView.m b/MathPad/MPExpressionView.m
index 0897b64..4a37a62 100644
--- a/MathPad/MPExpressionView.m
+++ b/MathPad/MPExpressionView.m
@@ -14,9 +14,6 @@
 
 #import "MPSumFunction.h"
 
-@interface MPExpressionView ()
-@end
-
 @implementation MPExpressionView
 
 #pragma mark Creation Methods
@@ -50,12 +47,20 @@
 
 - (void)initializeObjects
 {
-    MPExpressionStorage *expressionStorage = [[MPExpressionStorage alloc] initWithSymbols:@[@"12345", [[MPSumFunction alloc] init]]];
+    MPExpressionStorage *expressionStorage = [[MPExpressionStorage alloc] initWithSymbols:@[@"12 345", [[MPSumFunction alloc] init], [[MPSumFunction alloc] init]]];
     _expressionStorage = expressionStorage;
+    [self.expressionLayout setExpressionView:self];
 }
 
 #pragma mark Properties
 
+- (void)setExpressionStorage:(MPExpressionStorage *)expressionStorage
+{
+    [_expressionStorage.expressionLayout setExpressionView:nil];
+    _expressionStorage = expressionStorage;
+    [_expressionStorage.expressionLayout setExpressionView:self];
+}
+
 - (MPExpressionLayout *)expressionLayout
 {
     return self.expressionStorage.expressionLayout;