bitwise_copy(x, sizeof *x, 0, &value, field->n_bytes, sf->ofs, sf->n_bits);
bitwise_one ( &mask, field->n_bytes, sf->ofs, sf->n_bits);
bitwise_copy(x, sizeof *x, 0, &value, field->n_bytes, sf->ofs, sf->n_bits);
bitwise_one ( &mask, field->n_bytes, sf->ofs, sf->n_bits);