code: func (f *FSM) enterStateCallbacks(e *Event) {
  	if fn, ok := f.callbacks[cKey{f.current, callbackEnterState}]; ok {
  		fn(e)
  	}
  	if fn, ok := f.callbacks[cKey{"", callbackEnterState}]; ok {
  		fn(e)
  	}
  }
docs: // enterStateCallbacks calls the enter_ callbacks, first the named then the
  // general version.