static L<Pair<Symbol>> ai_triplesToBCPairs(final L<TripleWeb> l) {
  if (l == null) null;
  class TTBCList extends LazyList<Pair<Symbol>> {
    final int n = l(l);
    public int size() { ret n; }
    public Pair<Symbol> get(int i) {
      TripleWeb w = syncGet(l, i);
      ret w == null ? null : pair(w.b, w.c);
    }
  }
  ret new TTBCList;
}