Let tprod_tr m1 n1 (A :'M[F]_(m1, 1 + n1)) m2 n2 (B :'M[F]_(m2, n2)) :
tprod A B =  row_mx (trow (lsubmx A)^T B^T)^T (tprod (rsubmx A) B).

Lemma mxtrace_prod m n (A :'M[F]_(m)) (B :'M[F]_(n)) :
\tr (tprod A B) =  \tr A * \tr B.