If there is a way to define a "local" Ltac expresion which I can use to proof a lemma but not visible outside?
Lemma Foo ... Proof. Ltac ll := ... destrict t. - reflexivity. - ll. - ll. Qed. (* ll should not be visible here *)
You can use a section:
Section Foo. Ltac solve := exact I. Lemma Foo : True. Proof. solve. Qed. End Foo.