Latex counters: modifying \numberwithin?

[honestrosewater]

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:

Proof [text]
(1) [formula] assumption
(2) [formula] assumption
(3) [formula] MP(1, 2)
...

Proof [text]
(1) [formula] assumption
(2) [formula] MT(repl, 1)
...


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?

[diazona]

Here's the definition of the proof environment from amsthm.sty:
\newenvironment{proof}[1][\proofname]{\par
\pushQED{\qed}%
\normalfont \topsep6\p@\@plus6\p@\relax
\trivlist
\item[\hskip\labelsep
\itshape
#1\@addpunct{.}]\ignorespaces
}{%
\popQED\endtrivlist\@endpefalse
}
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.