idx const & first_idx=ex_to_lorentzidx(g.seq[0]);
idx const & second_idx=ex_to_lorentzidx(g.seq[1]);
// g_{mu,mu} should have been contracted in simp_lor::eval()
idx const & first_idx=ex_to_lorentzidx(g.seq[0]);
idx const & second_idx=ex_to_lorentzidx(g.seq[1]);
// g_{mu,mu} should have been contracted in simp_lor::eval()