code: function _get_referenced_subgraphs_by_filter(filter){
      var anchor = this;
  
      var ret = [];
      each(anchor._referenced_subgraphs, function(g){
  	var res = filter(g);
  	if( res && res === true ){
  	    ret.push(g);
  	}
      });
  
      return ret;
  }
docs: Get a sublist of referenced subgraphs using the filter
  function. The filter function take a single subgraph as an
  argument, and adds it to the return list if it evaluates to true.
  
  @name get_referenced_subgraphs_by_filter
  @function
  @param {Function} filter - function described above
  @returns {Array} list of passing subgraphs