docs: function (event, callback) {
          
              // (To learn how the handlers object works, see inline comment at Crafty.bind)
  
              //optimization for 1 entity
              if (this.length === 1) {
                  if (!handlers[event]) handlers[event] = {};
                  var h = handlers[event];
  
                  if (!h[this[0]]) h[this[0]] = []; //init handler array for entity
                  h[this[0]].push(callback); //add current callback
                  return this;
              }
  
              this.each(function () {
                  //init event collection
                  if (!handlers[event]) handlers[event] = {};
                  var h = handlers[event];
  
                  if (!h[this[0]]) h[this[0]] = []; //init handler array for entity
                  h[this[0]].push(callback); //add current callback
              });
              return this;
          }