code: function( event ) { //jshint ignore:line
          for ( var i = 0; i < preStepLeavePlugins.length; i++ ) {
              var thisLevel = preStepLeavePlugins[ i ];
              if ( thisLevel !== undefined ) {
                  for ( var j = 0; j < thisLevel.length; j++ ) {
                      if ( thisLevel[ j ]( event ) === false ) {
  
                          // If a plugin returns false, the stepleave event (and related transition)
                          // is aborted
                          return false;
                      }
                  }
              }
          }
      }
docs: Called at beginning of goto(), to execute all preStepLeave plugins.