theorem Th144:
  for y,w,z,v,x holds ((w | (z | (x | v))) | (((x | (y | (y | y))
  ) | z) | ((v | v) | z))) = z | (x | v)
proof
  let y,w,z,v,x;
  (z | (x | v)) | (z | (x | v)) = ((x | (y | (y | y))) | z) | ((v | v) | z
  ) by Th73;
  hence thesis by Th133;
end;
