sub print_to_file
  { my( $t, $filename)= (shift, shift);
    open( TWIG_PRINT_TO_FILE, ">$filename") or croak "cannot create file $filename: $!";
    $t->print( \*TWIG_PRINT_TO_FILE, @_);
    close TWIG_PRINT_TO_FILE;
    return $t;
  }