code: function (layers) {
  		if (!layers) {
  			layers = this._topClusterLevel.getAllChildMarkers();
  		} else if (layers instanceof L.MarkerClusterGroup) {
  			layers = layers._topClusterLevel.getAllChildMarkers();
  		} else if (layers instanceof L.LayerGroup) {
  			layers = layers._layers;
  		} else if (layers instanceof L.MarkerCluster) {
  			layers = layers.getAllChildMarkers();
  		} else if (layers instanceof L.Marker) {
  			layers = [layers];
  		} // else: must be an Array(L.Marker)|Map(L.Marker)
  		this._flagParentsIconsNeedUpdate(layers);
  		this._refreshClustersIcons();
  
  		// In case of singleMarkerMode, also re-draw the markers.
  		if (this.options.singleMarkerMode) {
  			this._refreshSingleMarkerModeMarkers(layers);
  		}
  
  		return this;
  	}
docs: Updates the icon of all clusters which are parents of the given marker(s).
  In singleMarkerMode, also updates the given marker(s) icon.
  @param layers L.MarkerClusterGroup|L.LayerGroup|Array(L.Marker)|Map(L.Marker)|
  L.MarkerCluster|L.Marker (optional) list of markers (or single marker) whose parent
  clusters need to be updated. If not provided, retrieves all child markers of this.
  @returns {L.MarkerClusterGroup}