joint.all.js
function undirEdgeId(u, v) {
    return u < v
      ? u.toString().length + ':' + u + '-' + v
      : v.toString().length + ':' + v + '-' + u;
  }
function undirEdgeId(u, v) {
    return u < v
      ? u.toString().length + ':' + u + '-' + v
      : v.toString().length + ':' + v + '-' + u;
  }
/*
Generate an ID that can be used to represent any undirected edge that is
incident on `u` and `v`.