for A, B, C, D, E being non empty set
for f1, f2 being Function of [:A,B,C,D:],E st ( for a being Element of A
for b being Element of B
for c being Element of C
for d being Element of D holds f1 . (a,b,c,d) = f2 . (a,b,c,d) ) holds
f1 = f2