static S rules_unfoldIndented(S s, S prefix) {

  if (containsTabs(s)) warn("Tabs in input");

  s = s.replace("\t", "  ");

  L<S> lines = toLines(s);

  for i over lines: {

    S line = lines.get(i);

    S trimmed = trim(line);

    if (empty(trimmed)) continue;

    int n = getIndent(line);

    int j = i+1;

    bool any = false;

    while (j < l(lines)) {

      if (emptyAfterTrim(lines.get(j))) ++j;

      else if (getIndent(lines.get(j)) > n) { ++j; any = true; }

      else break;

    }

    if (!any) continue;

    lines.set(i, "");

    while (i+1 < j) {

      ++i;

      S l = lines.get(i);

      if (!emptyAfterTrim(l))

        lines.set(i, rep(' ', getIndent(l)) + prefix

          + "(" + trimmed + ") & " + trim(l));

    }

  }

  ret fromLines(lines);

}