let L be non empty satisfying_Sh_1 ShefferStr ; :: thesis: for x, y, z, u being Element of L holds (x | y) | (z | u) = (u | z) | (y | x)
let x, y, z, u be Element of L; :: thesis: (x | y) | (z | u) = (u | z) | (y | x)
(x | y) | (z | u) = (x | y) | (u | z) by Th33;
hence (x | y) | (z | u) = (u | z) | (y | x) by Th37; :: thesis: verum