theorem :: DOMAIN_1:23

for X1, X2 being non empty set
for A1 being Subset of X1
for A2 being Subset of X2 holds [:A1,A2:] = { [x1,x2] where x1 is Element of X1, x2 is Element of X2 : ( x1 in A1 & x2 in A2 ) }