theorem :: SETWOP_2:8

for C, D being non empty set
for B being Element of Fin C
for e being Element of D
for F being BinOp of D
for f being Function of C,D st F is commutative & F is associative & F is having_a_unity & e = the_unity_wrt F & f .: B = {e} holds
F $$ (B,f) = e