theorem Th18:
  for X being BCI-Algebra_with_Condition(S) holds the ExternalDiff
  of X is having_a_unity
proof
  let X be BCI-Algebra_with_Condition(S);
  reconsider e=0.X as Element of X;
  e is_a_unity_wrt the ExternalDiff of X by Th16;
  hence thesis by SETWISEO:def 2;
end;
