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);
}