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 | ((y | y) | y)) = w
let y, w be Element of L; :: thesis: (w | w) | (w | ((y | y) | y)) = w
w | w = w | ((y | y) | y) by Th70;
hence (w | w) | (w | ((y | y) | y)) = w by SHEFFER1:def 13; :: thesis: verum