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 | p) | ((q | q) | p)
let p, q be Element of L; :: thesis: p = (q | p) | ((q | q) | p)
(q | q) | (q | q) = q by SHEFFER1:def 13;
hence p = (q | p) | ((q | q) | p) by Th91; :: thesis: verum