theorem Th74:
  for q,p,x holds (x | p) | ((q | q) | p) = ((p | ((x | x) | q)) |
  (p | ((x | x) | q)))
proof
  let q,p,x;
  (x | x) | (x | x) = x by SHEFFER1:def 13;
  hence thesis by SHEFFER1:def 15;
end;
