code: function( subClass, superClass )
      {
          function ctor() {} // wrapper to act as a constructor
  
          ctor.prototype     = superClass.prototype;
          subClass.super     = superClass.prototype;
          subClass.prototype = new ctor();
  
          /** @override */
          subClass.prototype.constructor = subClass;
      }
docs: convenience method to inherit prototypes, this can be used to
  inherit in an OOP-style pattern, e.g. :
  
  extend( Bus, Vehicle ) will apply the properties of the Vehicle prototype onto
  the Bus prototype. As thus, creating a new instance of Bus will also provide all
  properties and methods of Vehicle
  
  adapted from source by The Closure Library Authors
  
  @param {!Function} subClass reference to the prototype that will inherit from superClass
  @param {!Function} superClass reference to the prototype to inherit from