for A1, A2, B1, B2, C1, C2 being category
for F1 being Functor of A1,B1
for F2 being Functor of A2,B2
for G1 being Functor of B1,C1
for G2 being Functor of B2,C2 st F1 is covariant & G1 is covariant & F2 is covariant & G2 is covariant holds
(G1 [x] G2) (*) (F1 [x] F2) = (G1 (*) F1) [x] (G2 (*) F2)