arangosh> var rel = graph_module._relation("isCustomer", ["shop"], ["customer"]);
arangosh> graph._extendEdgeDefinitions(rel);
arangosh> graph;
{[Graph]
"isCustomer" : [ArangoCollection 15619, "isCustomer" (type edge, status loaded)],
"shop" : [ArangoCollection 15615, "shop" (type document, status loaded)],
"customer" : [ArangoCollection 15617, "customer" (type document, status loaded)],
"pet" : [ArangoCollection 15609, "pet" (type document, status loaded)]
}