existential instantiation and generalization in coq

Question

Can someone please give me a simple example of existential instantiation and existential generalization in Coq? When I want to prove exists x, P, where P is some Prop that uses x, I often want to name x (as x0 or some such), and manipulate P. Can this be one in Coq?

Answer 1

If you're going to prove the existential directly and not through a lemma, you can use eapply ex_intro. This introduces an existential variable (written ?42). You can then manipulate the term. To complete the proof, you need to eventually provide a way to construct a value for that variable. You can do this explicitly with the instantiate tactic, or implicitly through tactics such as eauto.

Beware that it is often cumbersome to work with existential variables. Many tactics assume that all terms are instantiated and may hide existentials in subgoals; you'll only find out when Qed tells you “Error: Attempt to save an incomplete proof”. You should only use existential variables when you have a plan to instantiate them soon.

Here's a silly example that illustrates the use of eapply.

Goal exists x, 1 + x = 3.
Proof.                        (* ⊢ exists x, 1 + x = 3 *)
  eapply ex_intro.            (* ⊢ 1 + ?42 = 3 *)
  simpl.                      (* ⊢ S ?42 = 3 *)
  apply f_equal.              (* ⊢ ?42 = 2 *)
  reflexivity.                (* proof completed *)
Qed.