// first file is assumed to be LHS
static Map<S, LS> lcMergeStringsNotOccurringOnLHS(Map<S, LS> encodings) {
  Set<S> lhs = asSet(firstValue(encodings));
  ret mapValues(encodings, l -> {
    if (l == firstValue(encodings)) ret l;
    l = cloneList(l);
    int i = 0;
    while (i < l(l)-1) {
      int j = i;
      while (j < l(l) && !lhs.contains(l.get(j))) ++j;
      if (j > i+1)
        replaceSublist(l, i, j, ll(joinSubList(l, i, j)));
      i++;
    }
    ret l;
  });
}

Began life as a copy of #1028860