theorem Th17:
  [{}.A, {}.A] is_a_unity_wrt FinPairUnion A
proof
  now
    let x be Element of [:Fin A, Fin A:];
    thus FinPairUnion A.([{}.A, {}.A], x) = [{}.A, {}.A] \/ x by Def6
      .= x by MCART_1:21;
  end;
  hence thesis by BINOP_1:4;
end;
