static WebNode web_findBackwardRelated(WebNode node, S relationLabel) {
  ret web_findBackwardRelated(node, node.web.parseLabel(relationLabel));
}

static WebNode web_findBackwardRelated(WebNode node, Lisp relationLabel) {
  Web web = node.web;
  for (Pair<WebNode> p : keys(web.relations))
    if (p.b == node) {
      WebNode rel = web.relations.get(p);
      if (rel != null && rel.hasLabel(relationLabel)) ret p.a;
    }
  null;
}

static WebNode web_findBackwardRelated(WebNode node) {
  Web web = node.web;
  for (Pair<WebNode> p : keys(web.relations))
    if (p.b == node)
      ret p.a;
  null;
}