static Pair<IntRange, O> findUsingWordTree(L<S> tok, Map<S, O> tree) {
  ret findUsingWordTree(tok, tree, 0);
}

// tok = CNC
// returns (index range C to N, tree leaf)
static Pair<IntRange, O> findUsingWordTree(L<S> tok, Map<S, O> tree, int i) {
  if (tree == null) null;
  O best = null; // best leaf
  int bestIndex = 0; // best index
  int n = l(tok);
  
  i |= 1; // Proceed to C token
  while (i <= n) {
    int j = i;
    while true {
      O def = tree.get("<default>");
      if (def != null) { best = def; bestIndex = j; }
      
      if (j >= n) break;
      O t = tree.get(tok.get(j));
      j += 2;
      if (t == null) break;
      if (!t instanceof Map) ret pair(intRange(i, j-1), t); // leaf!
      tree = (Map) t;
    }
    if (best != null) ret pair(intRange(i, bestIndex), best);
    i += 2;
  }
  
  null;
}