title: how to add new method in interface in golang
body: <p>I have a golang interface I</p>
  
  <pre><code>type I interface {
      A()
      B()
  }
  </code></pre>
  
  <p>This interface is an element of <code>type S struct</code>. Now I want to add a function C() to this interface which will be called an object of type S.
  But this interface is implemented by many other types(for ex: T).
  And on compiling I get an error as <code>T does not implement C()</code>.</p>
  
  <p>One workaround for this is add a dummy implementation of C() in T which just returns a value of the T's return type.</p>
  
  <p>Is there any better way to do this?</p>