if (ex_to<numeric>(it->coeff).is_negative())
neg_powers.push_back(recombine_pair_to_ex(expair(it->rest, -(it->coeff))));
else
if (ex_to<numeric>(it->coeff).is_negative())
neg_powers.push_back(recombine_pair_to_ex(expair(it->rest, -(it->coeff))));
else