title: Turn a NSDecimalNumber negative
body: <p>I am looking for a way to turn a <code>NSDecimalNumber</code> negative by multiplying by <code>-1</code>.</p>
  
  <pre><code>/* decNumber is the one I would like to turn negative */
  NSDecimalNumber *decNumber = [values objectAtIndex:billIndex];
  
  NSDecimalNumber *minusOne = [[NSDecimalNumber alloc] initWithInt: -1];
  finalValue = [[NSDecimalNumber alloc] initWithDecimal: [[decNumber decimalNumberByMultiplyingBy: minusOne] decimalValue]];
  </code></pre>
  
  <p>This works but it feels like it's just too much for such a simple logic. Can you think of a better way to achieve this?</p>