code: function(C, action, thisp) {
  		var results = [];
  		action = action || gui.Combo.identity;
  		C.$subclasses.forEach(function(sub) {
  			results.push(action.call(thisp, sub));
  		}, thisp);
  		return results;
  	}
docs: Return superclass. If action is provided, return an array of the results
  of executing the action for each subclass with the subclass as argument.
  @param {function} C constructor
  @param @optional {function} action
  @param @optional {object} thisp
  @returns {Array<gui.Class|object>}