I am writing up some formal proofs using the proof and align environments. Each line in the proof needs to be numbered and will contain a formula and a statement referring to earlier lines in the proof that were used to derive it. I want to reset this counter for each proof. It should look something like this:

What's the easiest way to do this? Should I define a new proof counter and modify \numberwithin to reset the equation counter when proof is incremented? (I don't know how to do this.) Or should I skip using equation and just define my own counter that will increment with each new line in align? (I don't know how to do this either.)

Edit: Oh, duh, I'll just add \setcounter manually where I want it reset. Is there a way to get the proof environment to do this automatically?

You could always just repeat this in your document preamble (or put it in a style file of your own) and add in a \setcounter statement in the appropriate place.