code: public void addLayerWithFilter(String resultTag, String serverLayerId, String filter) {
  		// note: layer filter specification will be overwritten if it already
  		// exists
  		layerFilters.put(resultTag, new LayerFilterSpecification(serverLayerId, filter));
  	}
docs: Add a layer with an optional filter expression which should be applied on the given layer.
  <p/>
  If the filter contains a geometry, then this needs to be in layer CRS, it is not converted!
  
  @param resultTag tag to make the distinction in the response object between the features for the same
  serverLayerId but a different filter (e.g. client layer id)
  @param serverLayerId server layerId layer to set this filter on
  @param filter filter expression for the specified layer, can be null (==true filter)no client layer specific
  filtering
  
  @since 1.10.0