let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for p, y being Element of L holds (y | p) | ((y | y) | p) = (p | p) | (y | p)
let p, y be Element of L; :: thesis: (y | p) | ((y | y) | p) = (p | p) | (y | p)
p | (y | p) = (y | y) | p by Th119;
hence (y | p) | ((y | y) | p) = (p | p) | (y | p) by Th119; :: thesis: verum