theorem :: MCART_1:81

for X1, X2, X3, X4, Z being set st ( for z being object holds
( z in Z iff ex x1, x2, x3, x4 being object st
( x1 in X1 & x2 in X2 & x3 in X3 & x4 in X4 & z = [x1,x2,x3,x4] ) ) ) holds
Z = [:X1,X2,X3,X4:]