static void print_setPrefixForThread(fS prefix) {
  interceptPrintInThisThread(empty(prefix) ? null
    : new F1<S, Bool>() {
      bool beginningOfLine = true;
      Bool get(S s) {
        if (empty(s)) false;
        if (beginningOfLine) print_raw(prefix);
        bool nl = s.endsWith("\n");
        if (nl) s = dropLast(s);
        s = s.replace("\n", "\n" + prefix);
        print_raw(s);
        if (nl) print_raw("\n");
        beginningOfLine = nl;
        false;
      }
    });
}