let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z, u being Element of L holds (x | (y | z)) | (x | (u | (y | x))) = x

let x, y, z be Element of L; :: thesis: for u being Element of L holds (x | (y | z)) | (x | (u | (y | x))) = x

(x | (y | z)) | (y | x) = x by Th25;

hence for u being Element of L holds (x | (y | z)) | (x | (u | (y | x))) = x by Th29; :: thesis: verum

let x, y, z be Element of L; :: thesis: for u being Element of L holds (x | (y | z)) | (x | (u | (y | x))) = x

(x | (y | z)) | (y | x) = x by Th25;

hence for u being Element of L holds (x | (y | z)) | (x | (u | (y | x))) = x by Th29; :: thesis: verum