code: function addArrowMeta(graph, head, tail, meta) {
  	var entry = graph.arrowsMeta.get(head);
  
  	if (entry) {
  		var arrowMeta = entry.get(tail);
  		if (!arrowMeta) {
  			arrowMeta = {};
  		}
  		entry.set(tail, canAssign(arrowMeta, meta));
  	} else {
  		entry = new Map();
  		entry.set(tail, meta);
  		graph.arrowsMeta.set(head, entry);
  	}
  }
docs: Helpers