joint.all.js
function incidenceId(u, v) {
  return u < v ?  u.length + ':' + u + '-' + v : v.length + ':' + v + '-' + u;
}
function incidenceId(u, v) {
  return u < v ?  u.length + ':' + u + '-' + v : v.length + ':' + v + '-' + u;
}
/*
This id can be used to group (in an undirected manner) multi-edges
incident on the same two nodes.