Substitute argument of `fix` in proof

问题

This question is probably trivial, but I'm stuck on it since yesterday and I couldn't find the relevant keyword to search for.

Consider the following:

Fixpoint mfp (t: nat*nat) := fst t.

Lemma ml: forall (t: nat*nat), mfp t = fst t.
Proof.
  intros.
  unfold mfp.
  (* substitute t0 with t in lhs *)
  reflexivity.
Qed.

After unfolding mfp, I have to prove (fix mfp (t0 : nat * nat) : nat := fst t0) t = fst t which trivially holds, yet I don't know how to tell Coq "Do the substitution of t0 by t".

Do you know how to do that substitution ?

来源：https://stackoverflow.com/questions/52382140/substitute-argument-of-fix-in-proof