sclass _PrintFlexIndent extends F1<S, Bool> {
  F1<S, S> makeIndent;
  bool beginningOfLine = true;
  
  *() {}
  *(F1<S, S> *makeIndent) {}
  
  Bool get(S s) {
    if (empty(s)) false;
    S prefix = null;
    if (beginningOfLine) {
      if (prefix == null) prefix = unnull(callF(makeIndent, s));
      print_raw(prefix);
    }
    bool nl = s.endsWith("\n");
    if (nl) s = dropLast(s);
    if (contains(s, '\n')) {
      if (prefix == null) prefix = unnull(callF(makeIndent, s));
      if (nempty(prefix)) s = s.replace("\n", "\n" + prefix);
    }
    print_raw(s);
    if (nl) print_raw("\n");
    beginningOfLine = nl;
    false;
  }
}

Began life as a copy of #1008514