for R1, R2, R3 being non empty RelStr
for f1 being Function of R1,R3 st f1 is isomorphic holds
for f2 being Function of R2,R3 st f2 = f1 & f2 is isomorphic holds
RelStr(# the carrier of R1, the InternalRel of R1 #) = RelStr(# the carrier of R2, the InternalRel of R2 #)