theorem Th54: :: MCART_1:68

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