title: Faster way for NSMutableDictionary check
body: <p>I'm doing some app optimizations and in big loops the checks on NSMutableDictionary's are fairly taxing:</p>
  
  <pre><code>if ([self.variantBufferSequence objectForKey:[variant valueForKey:@"model_no"]] == nil) {
      [self.variantBufferSequence setObject:[NSMutableDictionary new] forKey:[variant valueForKey:@"model_no"]];
  }
  </code></pre>
  
  <p>Is there a lighter way to check the existence of the entry (<code>objectForKey</code>) in the <code>if</code>-statement?</p>