sS tok_genRecordTransformable(S name, LPairS fields) {
  ret "public O transformUsing(IF1 f) { " +
    (empty(fields) ? "this;" : "ret new " + name + "("
    + joinWithComma(map(pairsB(fields), field -> "f.get(" + field + ")"))
    + ");")
    + " }\n";
}