let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for y, q, w being Element of L holds ((w | q) | ((y | y) | y)) | ((w | q) | (w | q)) = ((w | w) | (w | q)) | ((q | q) | (w | q))
let y, q, w be Element of L; :: thesis: ((w | q) | ((y | y) | y)) | ((w | q) | (w | q)) = ((w | w) | (w | q)) | ((q | q) | (w | q))
(w | q) | (w | q) = (w | q) | ((y | y) | y) by Th70;
hence ((w | q) | ((y | y) | y)) | ((w | q) | (w | q)) = ((w | w) | (w | q)) | ((q | q) | (w | q)) by SHEFFER1:def 15; :: thesis: verum