for S1, S2, S3 being non empty ManySortedSign
for f1, g1, f2, g2 being Function st S1,S2 are_equivalent_wrt f1,g1 & S2,S3 are_equivalent_wrt f2,g2 holds
S1,S3 are_equivalent_wrt f2 * f1,g2 * g1