static BigInt combinationsForPositionalParser(int length, IF1<Int, L<Int>> skipsFromPosition) {
  Map<Int, BigInt> combinationsFromPosition = autoTreeMap(() -> bigint(1));
  for (int i = positions-1; i >= 0; i--) {
    BigInt combinations = bigint(0);
    for (int skip : skipsFromPosition(i))
      combinations = plus(combinations, combinationsFromPosition.get(i+skip));
    combinationsFromPosition.put(i, combinations);
  }
  ret combinationsFromPosition.get(0);
}