static PosNegMatrix<LS> posNegMatrixForNGrams(Cl<S> posNgrams, Cl<S> negNGrams, Cl<S> pos, Cl<S> neg) {
  PosNegMatrix<LS> matrix = posNegMatrixOfLists();
  for (S s : pos) {
    int score = substringsScoreICPosNeg(s, posNgrams, negNGrams);
    bool ok = score > 0;
    (ok ? matrix.pos : matrix.falseNeg).add(s);
  }
  for (S s : neg) {
    int score = substringsScoreICPosNeg(s, posNgrams, negNGrams);
    bool ok = score < 0;
    (ok ? matrix.neg : matrix.falsePos).add(s);
  }
  ret matrix;
}

Began life as a copy of #1027472