sclass _PrintIndent extends F1<S, Bool> {
  S prefix;
  bool beginningOfLine = true;
  
  *() {}
  *(S *prefix) {}
  
  Bool get(S s) {
    if (empty(s)) false;
    bool nl = s.endsWith("\n");
    if (nl) s = dropLast(s);
    s = s.replace("\n", "\n" + prefix);
    print_raw(beginningOfLine ? prefix + s : s);
    if (nl) print_raw("\n");
    beginningOfLine = nl;
    false;
  }
}