let S, T be non empty RelStr ; :: thesis: for x, y being Element of [:S,T:] holds
( x is_<=_than {y} iff ( x `1 is_<=_than {(y `1)} & x `2 is_<=_than {(y `2)} ) )
let x, y be Element of [:S,T:]; :: thesis: ( x is_<=_than {y} iff ( x `1 is_<=_than {(y `1)} & x `2 is_<=_than {(y `2)} ) )
thus ( x is_<=_than {y} implies ( x `1 is_<=_than {(y `1)} & x `2 is_<=_than {(y `2)} ) ) :: thesis: ( x `1 is_<=_than {(y `1)} & x `2 is_<=_than {(y `2)} implies x is_<=_than {y} )

proof

A1: the carrier of [:S,T:] = [: the carrier of S, the carrier of T:] by YELLOW_3:def 2;
then A2: x = [(x `1),(x `2)] by MCART_1:21;
y = [(y `1),(y `2)] by A1, MCART_1:21;
then A3: [(y `1),(y `2)] in {y} by TARSKI:def 1;
assume for b being Element of [:S,T:] st b in {y} holds
x <= b ; :: according to LATTICE3:def 8 :: thesis: ( x `1 is_<=_than {(y `1)} & x `2 is_<=_than {(y `2)} )
then A4: x <= [(y `1),(y `2)] by A3;

hereby :: according to LATTICE3:def 8 :: thesis: x `2 is_<=_than {(y `2)}

let b be Element of S; :: thesis: ( b in {(y `1)} implies x `1 <= b )
assume b in {(y `1)} ; :: thesis: x `1 <= b
then b = y `1 by TARSKI:def 1;
hence x `1 <= b by A4, A2, YELLOW_3:11; :: thesis: verum

end;

let b be Element of T; :: according to LATTICE3:def 8 :: thesis: ( not b in {(y `2)} or x `2 <= b )
assume b in {(y `2)} ; :: thesis: x `2 <= b
then b = y `2 by TARSKI:def 1;
hence x `2 <= b by A4, A2, YELLOW_3:11; :: thesis: verum

end;

assume that
A5: for b being Element of S st b in {(y `1)} holds
x `1 <= b and
A6: for b being Element of T st b in {(y `2)} holds
x `2 <= b ; :: according to LATTICE3:def 8 :: thesis: x is_<=_than {y}
let b be Element of [:S,T:]; :: according to LATTICE3:def 8 :: thesis: ( not b in {y} or x <= b )
assume b in {y} ; :: thesis: x <= b
then A7: b = y by TARSKI:def 1;
then b `2 in {(y `2)} by TARSKI:def 1;
then A8: x `2 <= b `2 by A6;
b `1 in {(y `1)} by A7, TARSKI:def 1;
then x `1 <= b `1 by A5;
hence x <= b by A8, YELLOW_3:12; :: thesis: verum