let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for p, x, y being Element of L holds ((y | (x | x)) | (y | (x | x))) | (p | (p | p)) = (x | x) | y
let p, x, y be Element of L; :: thesis: ((y | (x | x)) | (y | (x | x))) | (p | (p | p)) = (x | x) | y
((x | x) | y) | ((x | x) | y) = (y | (x | x)) | (y | (x | x)) by SHEFFER1:def 15;
hence ((y | (x | x)) | (y | (x | x))) | (p | (p | p)) = (x | x) | y by Th71; :: thesis: verum