code: function _get_referenced_subgraph_by_id(iid){
      var anchor = this;
  
      var ret = null;
      each(anchor._referenced_subgraphs, function(g){
  	if( g.id() === iid ){
  	    ret = g;
  	}
      });
  
      return ret;
  }
docs: Get a referenced_subgraph with a certain ID.
  
  @name get_referenced_subgraph_by_id
  @function
  @param {String} iid - referenced_individual ID to look for
  @returns {Object|null} referenced_subgraph with that ID