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;
}
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.
/ * This id can be used to group ( in an undirected manner ) multi - edges incident on the same two nodes .