An EL in a command binding cannot access "event" object

steps to reproduce

1. load the zul
2. click the button "event"

result

it prints "true" with the event object

expected result

it prints "false" for event is not null.

debug info

• From a specification view, an EL expression should be able to access all implicit objects according to http://books.zkoss.org/wiki/ZUML%20Reference/EL%20Expressions/Implicit%20Objects%20(Predefined%20Variables)
• we can pass the event as a parameter but cannot access it in an EL expression. (inconsistency)
• related code

  				command = (String) eval.getValue(null, comp, ((CommandBindingImpl)_commandBinding).getCommand());
  				if(!Strings.isEmpty(command)){//avoid the execution of a empty command.
  					
  					//ZK-1032 Able to wire Event to command method
  					Map<String,Object> implicit = null;
  					if(_implicitContributor!=null){
  						implicit = _implicitContributor.contirbuteCommandObject(BinderImpl.this,_commandBinding,event);
  					}
  ...					
  				}
  

  only _implicitContributor contains event, when evaluating a command name, the context doesn't contain event.