[AUCTeX] Replace \begin{exer} with %%Exercise n\par\begin{exer}, n a counter

[David Arnold]

All,

I'd like to replace each occurrence

\begin{exer}

with

%%Exercise n
\begin{exer}

where n is a counter that is incremented on each replacement. How can I 
do this in Emacs-Auctex?

Thanks.