// tok = CNC
// returns (index, tree leaf)
static Pair<Int, O> matchStartUsingWordTree(L<S> tok, Map<S, O> tree) {
  O best = null; // best leaf
  int bestIndex = 0; // best index
  
  int j = 1;
  while (j < l(tok)) {
    O def = tree.get("<default>");
    if (def != null) { best = def; bestIndex = j; }
   
    O t = tree.get(tok.get(j));
    j += 2;
    if (t == null) break;
    if (!t instanceof Map) ret pair(j, t); // leaf!
    tree = (Map) t;
  }
  
  ret pair(bestIndex, best);
}