title: Why can't I make a postfix operator named '!'
body: <p>I try to make a postfix operator, but it gives me the error "Cannot declare a custom postfix '!' operator"</p>
  
  <pre><code>postfix operator !
  postfix func !(lhs: UInt) -&gt; Double {
      var res = 1.0
  
      for multiplier in 1...lhs {
          res *= Double(multiplier)
      }
  
      return res
  }
  </code></pre>
  
  <p>I did not expect the error. How can I fix this?</p>