declare C { mode=(2,2), latex="C" }

declare S { mode=(2,2), latex="S" }

declare T { mode=(2,2), latex="T" }

declare nbar { mode=2, diagonal=true, latex="\bar{n}" }

C_pqrs = 1/2 * sum_tu(nbar_t * nbar_u * S_pqtu * T_turs);
