let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y being Element of L holds (x | y) | y = y | (x | x)
let x, y be Element of L; :: thesis: (x | y) | y = y | (x | x)
set X = x;
set Y = y;
y | (x | y) = (x | y) | y by Th20;
hence (x | y) | y = y | (x | x) by Th42; :: thesis: verum