title: Flow type, what do `+` and `-` do in a generic declaration?
body: <p>Something like:</p>
  
  <pre><code>declare class $ReadOnlyArray&lt;+T&gt;
  declare type React$Element&lt;+ElementType: React$ElementType&gt;
  interface $Iterator&lt;+Yield,+Return,-Next&gt;
  </code></pre>
  
  <p>Not the kind that appears in front of properties, which is answered here <a href="https://stackoverflow.com/questions/46338710/flow-type-what-does-the-symbol-mean-in-front-a-property">Flow type, What does the `+` symbol mean in front a property?</a></p>