Some small corrections

Added example expression (hardcoded for development)
Added example change for expressions (hardcoded for development)

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