let L be non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr ; :: thesis: for p, q being Element of L holds p = (((q | q) | (q | q)) | p) | ((q | q) | p)
let p, q be Element of L; :: thesis: p = (((q | q) | (q | q)) | p) | ((q | q) | p)
(p | p) | (p | ((q | q) | q)) = p by Th77;
hence p = (((q | q) | (q | q)) | p) | ((q | q) | p) by Th90; :: thesis: verum