title: Swift: masks as type properties
body: <p>my problem. I'm working with a NSEvent, which happens to have a var <strong>modifierFlags</strong> of type <strong>NSEventModifierFlags</strong>. </p>
  
  <p>I want to check whether the user had a modifier key pressed (Command), so basically I want to check if <strong>CommandKeyMask</strong> is on. </p>
  
  <p>What's the best/right way to do this in Swift ?</p>