let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for y, z, x being Element of L holds ((y | ((x | x) | z)) | (y | ((x | x) | z))) | ((x | y) | ((z | z) | y)) = y | ((x | x) | z)
let y, z, x be Element of L; :: thesis: ((y | ((x | x) | z)) | (y | ((x | x) | z))) | ((x | y) | ((z | z) | y)) = y | ((x | x) | z)
(y | ((x | x) | z)) | (y | ((x | x) | z)) = (x | y) | ((z | z) | y) by Th74;
hence ((y | ((x | x) | z)) | (y | ((x | x) | z))) | ((x | y) | ((z | z) | y)) = y | ((x | x) | z) by Th121; :: thesis: verum