theorem Th68: :: MCART_1:84

for X1, X2, X3, X4, Y1, Y2, Y3, Y4 being set st X1 c= Y1 & X2 c= Y2 & X3 c= Y3 & X4 c= Y4 holds
[:X1,X2,X3,X4:] c= [:Y1,Y2,Y3,Y4:]