let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for x, w, y, z being Element of L holds (((x | x) | w) | ((z | (y | (y | y))) | w)) | w = w | (x | z)
let x, w, y, z be Element of L; :: thesis: (((x | x) | w) | ((z | (y | (y | y))) | w)) | w = w | (x | z)
(w | (x | z)) | (w | (x | z)) = ((x | x) | w) | ((z | (y | (y | y))) | w) by Th75;
hence (((x | x) | w) | ((z | (y | (y | y))) | w)) | w = w | (x | z) by Th131; :: thesis: verum