for X1, X2, X3 being StackAlgebra
for F1, F2, G1, G2 being Function st F1,G1 form_isomorphism_between X1,X2 & F2,G2 form_isomorphism_between X2,X3 holds
F2 * F1,G2 * G1 form_isomorphism_between X1,X3