memcpy (drop_vars, vm->drop_vars, drop_cnt * sizeof *drop_vars);
sort (drop_vars, drop_cnt, sizeof *drop_vars,
compare_variables_given_ordering, &forward_positional_ordering);
memcpy (drop_vars, vm->drop_vars, drop_cnt * sizeof *drop_vars);
sort (drop_vars, drop_cnt, sizeof *drop_vars,
compare_variables_given_ordering, &forward_positional_ordering);