let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for z, y, x being Element of L holds (z | (x | y)) | y = y | ((x | x) | z)
let z, y, x be Element of L; :: thesis: (z | (x | y)) | y = y | ((x | x) | z)
((y | ((x | x) | z)) | (y | ((x | x) | z))) | ((x | y) | ((z | z) | y)) = (z | (x | y)) | y by Th123;
hence (z | (x | y)) | y = y | ((x | x) | z) by Th145; :: thesis: verum