reserve X,Y,x for set;
reserve A for non empty preBoolean set;

theorem
  for A, B being set holds Fin A \/ Fin B c= Fin (A \/ B)
proof
  let A, B be set;
  Fin A c= Fin (A \/ B) & Fin B c= Fin(A \/ B) by Th10,XBOOLE_1:7;
  hence thesis by XBOOLE_1:8;
end;
