static Web webFromTriples(S... l) {
  Web web = simpleWeb();
  for (int i = 0; i < l(l); i += 3) {
    S a = unnull(get(l, i)), b = unnull(get(l, i+1)), c = unnull(get(l, i+2));
    web_addRelation(a, c, b);
  }
  ret web;
}