Creating Coq tactic: how to use a newly generated name?

Question

I want to create a Coq tactic that looks something like the following. I assert a proposition named H, I prove that proposition, and then I use simpl within that proposition. The tactic would look something like this:

Tactic Notation "foo" :=
  assert (True) as H; try apply I;
  simpl in H.

However, instead of using the name H for this hypothesis, I want Coq to generate a new name for me. The problem is, how can I then use simpl in this hypothesis?

Tactic Notation "foo" :=
  assert (True) as ?; try apply I;
  simpl in (* what? *).

Is there a way that I can generate a hypothesis name, and then refer back to it within the same tactic?

Answer

You can use the fresh tactic for this. Here is an example:

Tactic Notation "foo" :=
  let H := fresh "H" in
  assert True as H; try apply I; simpl in H.

Goal False.
False
foo. (* H : True *)
H: True
False
foo. (* H, H0 : True *)
H, H0: True
False
Abort.