theorem Th155:
  for z,w,y,x holds w | x = (w | ((x | z) | (((x | (y | (y | y)))
  | (x | (y | (y | y)))) | w)))
proof
  let z,w,y,x;
  (x | (y | (y | y))) | (x | z) = x by Th134;
  hence thesis by Th148;
end;
