Internal Redesign:

- Combined MPExpression and MPMutableExpression
- Abstracted children of MPExpression into MPExpressionElement protocol
- Abstracted most of MPExpressionLayout and MPFunctionLayout into common superclass MPLayout

MathPad/MPSumFunction.m

@@ -12,7 +12,6 @@
 @implementation MPSumFunction
 
 #pragma mark Creation Methods
-
 - (instancetype)init
 {
     self = [super init];
@@ -27,8 +26,32 @@
     return self;
 }
 
-#pragma mark Working With the Expression Tree
+#pragma mark Properties
+- (void)setStartExpression:(MPExpression *)startExpression
+{
+    _startExpression.parent = nil;
+    _startExpression = startExpression;
+    _startExpression.parent = self;
+    [self didChangeChildAtIndex:0];
+}
 
+- (void)setTargetExpression:(MPExpression *)targetExpression
+{
+    _targetExpression.parent = nil;
+    _targetExpression = targetExpression;
+    _targetExpression.parent = self;
+    [self didChangeChildAtIndex:1];
+}
+
+- (void)setSumExpression:(MPExpression *)sumExpression
+{
+    _sumExpression.parent = nil;
+    _sumExpression = sumExpression;
+    _sumExpression.parent = self;
+    [self didChangeChildAtIndex:2];
+}
+
+#pragma mark Working With the Expression Tree
 - (NSUInteger)numberOfChildren
 {
     return 3;
@@ -48,36 +71,32 @@
     }
 }
 
-- (NSArray *)children
-{
-    return @[self.startExpression, self.targetExpression, self.sumExpression];
-}
-
 - (void)setChild:(MPExpression *)child
          atIndex:(NSUInteger)index
 {
     switch (index) {
             // TODO: Copy child?
         case 0:
-            self.startExpression.parent = nil;
             self.startExpression = child;
             break;
         case 1:
-            self.targetExpression.parent = nil;
             self.targetExpression = child;
             break;
         case 2:
-            self.sumExpression.parent = nil;
             self.sumExpression = child;
             break;
         default:
             return;
     }
-    child.parent = self;
+    [self didChangeChildAtIndex:index];
+}
+
+- (NSArray *)children
+{
+    return @[self.startExpression, self.targetExpression, self.sumExpression];
 }
 
 #pragma mark Evaluating Functions
-
 - (double)doubleValue
 {
 #warning Implementation
@@ -85,7 +104,6 @@
 }
 
 #pragma mark Working With Functions
-
 - (BOOL)isEqualToFunction:(MPFunction *)aFunction
 {
     if (![aFunction isKindOfClass:[MPSumFunction class]]) {
@@ -106,8 +124,13 @@
     return [NSString stringWithFormat:@"Sum(From: %@; To: %@; Using: %@)", self.startExpression, self.targetExpression, self.sumExpression];
 }
 
-#pragma mark - NSCopying
+- (NSUInteger)hash
+{
+#warning Unimplemented Method
+    return [super hash];
+}
 
+#pragma mark - NSCopying
 - (id)copyWithZone:(NSZone *)zone
 {
     MPSumFunction *copy = [[MPSumFunction allocWithZone:zone] init];