// return new rule text, add comments to comments
sS gazelle_processSquareBracketAnnotations(S ruleText, LS comments) {
  LS lines = tlft(ruleText);
  while (isSquareBracketed(last(lines)))
    comments.add(deSquareBracket(popLast(lines)));
    
  int out = -1;
  for i over lines: {
    S s = lines.get(i);
    PairS p = splitTrailingSquareBracketStuff(s);
    if (startsWith(s, "=>")) out = i;
    if (nempty(p.b))
      comments.add(
        (out >= 0 ? i == out && i == l(lines)-1 ? "out" : "out " + (i-out+1) : "in " + (i+1))
        + " = " + gazelle_canonicalizeLineType(p.b));
    lines.set(i, p.a);
  }
 
  ret lines_rtrim(lines);
}

Began life as a copy of #1021762