// the result indices are kind of nonsensical; we should make new pairs instead
static Map<S, L<Int>> lcRenumberTopLevelEncodings_simplify(Map<S, L<Int>> encodings) {
  new MultiSet<Int> ms;
  for (L<Int> l : values(encodings))
    ms.addAll(l);
  
  new LinkedHashMap<S, L<Int>> out;
  for (S file, L<Int> l : encodings) {
    new L<Int> l2;
    for i over l: {
      if (i > 0 && ms.get(l.get(i-1)) == 1
        && ms.get(l.get(i)) == 1) continue;
      l2.add(l.get(i));
    }
    out.put(file, l2);
  }
  ret out;
}

Began life as a copy of #1028771