static void web_addRelation(WebNode a, WebNode b, S rel) {
  if (a != null && b != null && rel != null)
    a.web.getRelation(a, b).addLabel(rel);
}

static void web_addRelation(Web web, S a, S b, S rel) {
  web_addRelation(web.node(a), web.node(b), rel);
}

static void web_addRelation(WebNode a, S rel, WebNode b) {
  web_addRelation(a, b, rel);
}