code: function ( clip, optionalRoot ) {
  
  		var root = optionalRoot || this._root,
  			rootUuid = root.uuid,
  
  			clipObject = typeof clip === 'string' ?
  				AnimationClip.findByName( root, clip ) : clip,
  
  			clipUuid = clipObject ? clipObject.uuid : clip,
  
  			actionsForClip = this._actionsByClip[ clipUuid ];
  
  		if ( actionsForClip !== undefined ) {
  
  			return actionsForClip.actionByRoot[ rootUuid ] || null;
  
  		}
  
  		return null;
  
  	}
docs: get an existing action