Added the MPExpressionTree Classes

MathPad/MPLayout.m

@@ -174,6 +174,11 @@
     return bounds;
 }
 
+- (BOOL)drawsChildrenManually
+{
+    return NO;
+}
+
 - (void)drawAtPoint:(NSPoint)point
 {
     NSAffineTransform *transform = [NSAffineTransform transform];
@@ -181,6 +186,13 @@
                         yBy:point.y];
     [transform concat];
     [self draw];
+    if (!self.drawsChildrenManually) {
+        for (NSUInteger index = 0; index < [self numberOfChildren]; index++) {
+            MPLayout *childLayout = [self childLayoutAtIndex:index];
+            NSPoint offset = [self offsetOfChildLayoutAtIndex:index];
+            [childLayout drawAtPoint:offset];
+        }
+    }
     [transform invert];
     [transform concat];
 }