theorem Th94:
  for z,w,x holds ((((x | x) | w) | ((z | z) | w)) | ((w | (x | z)
  ) | (w | (x | z)))) = (w | (x | z))
proof
  let z,w,x;
  (((w | w) | (w | (x | z))) | (((x | z) | (x | z)) | (w | (x | z)))) = (w
  | (x | z)) by Th89;
  hence thesis by Th93;
end;
