static Web webFromTriple(Triple<S> t) {
  ret webFromTriple(t, 0.2, 0.2, 0.8, 0.7);
}

static Web webFromTriple(Triple<S> t, double x1, double y1, double x2, double y2) {
  Web web = webFromTriples(t.a, t.b, t.c);
  web_setPosition(first(web.nodes), x1, y1);
  web_setPosition(second(web.nodes), x2, y2);
  ret web;
}