for A1, A2, A3 being category
for F being covariant Functor of A1,A2
for G being covariant Functor of A2,A3
for B1 being non empty subcategory of A1
for B2 being non empty subcategory of A2
for B3 being non empty subcategory of A3 st B1,B2 are_isomorphic_under F & B2,B3 are_isomorphic_under G holds
B1,B3 are_isomorphic_under G * F