let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for y, w being Element of L holds (w | w) | ((w | w) | ((y | y) | y)) = (y | y) | y
let y, w be Element of L; :: thesis: (w | w) | ((w | w) | ((y | y) | y)) = (y | y) | y
w | ((y | y) | y) = w | w by Th70;
hence (w | w) | ((w | w) | ((y | y) | y)) = (y | y) | y by Th92; :: thesis: verum