let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z being Element of L holds (x | (y | y)) | (x | (z | z)) = (x | (z | y)) | (x | (z | y))
let x, y, z be Element of L; :: thesis: (x | (y | y)) | (x | (z | z)) = (x | (z | y)) | (x | (z | y))
set X = x | (y | y);
(x | (y | y)) | (x | (z | z)) = (x | (y | y)) | (x | (z | (x | (y | y)))) by Th61;
hence (x | (y | y)) | (x | (z | z)) = (x | (z | y)) | (x | (z | y)) by Th64; :: thesis: verum