title: stopping a block enumeration fo a NSDictionary
body: <p>How do I stop an enumeration here if I am doing the following? I think the docs said that to set stop = TRUE. but when I do that inside the block it says that it isn't unassignable.</p>
  
  <pre><code> [self.visibleViewControllers_ enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
  
              if (CGRectIntersectsRect(visibleRect, viewRect)) {
                 //break here
              }
          }];
  </code></pre>