I would like to have the following feature: Imagine I have a text like this.
Theorem 1: Every integer has a successor.
Theorem 2: There are infinitely many integers.
Proof: We use Theorem 1 here.
What I want to achieve is a little note on the margin near Theorem 1 indicating (with a hyperlink) that Theorem 2 (or its proof) uses Theorem 1.
How does this work? (It is similar to backref, but I want to apply it to theorem-like environments and I want to get not just the page number where the theorem is used, but the precise environment itself.)
