theorem Th151:
  for z,p,y,x holds z | (x | p) = z | (p | ((x | (y | (y | y))) |
  (x | (y | (y | y)))))
proof
  let z,p,y,x;
  (((p | p) | z) | ((x | (y | (y | y))) | z)) | (((p | p) | z) | ((x | (y
  | (y | y))) | z)) = z | (p | ((x | (y | (y | y))) | (x | (y | (y | y))))) by
Th130;
  hence thesis by Th150;
end;
