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sclass WebNode {
  new L<S> labels;
  
  void addLabel(S label) {
    setAdd(labels, label);
  }
  
  bool hasLabel(S label) { ret labels.contains(label); }
}

sclass Web {
  new L<WebNode> nodes;
  Map<Pair<WebNode>, WebNode> relations = new HashMap;
  
  void relation(Lisp l) {
    if (l(l) != 2) ret;
    getRelation(l.s(0), l.s(1)).addLabel(l.head);
  }
  
  void relations(L<Lisp> l) {
    for (Lisp li : l) relation(li);
  }
  
  WebNode getRelation(S a, S b) {
    Pair<WebNode> p = pair(findNode(a), findNode(b));
    WebNode r = relations.get(p);
    if (r == null) relations.put(p, r = new WebNode);
    ret r;
  }
  
  WebNode findNode(S s) {
    for (WebNode n : nodes)
      if (n.labels.contains(s))
        ret n;
    ret newNode(s);
  }
  
  WebNode newNode(S... labels) {
    new WebNode n;
    for (S label : labels) n.addLabel(label);
    nodes.add(n);
    ret n;
  }
}