Rule of Theorem Introduction

Context
Natural deduction.

Definition
We may introduce, at any stage of a proof (citing "TI"), a theorem already proved, or a substitution instance of such a theorem, together with a reference to the theorem that is being cited.

Proof
This theorem is a corollary of the Rule of Sequent Introduction.

Application
Using this rule, we can use theorems that we have derived in order to shorten proofs which may otherwise be long and unwieldy.