\documentclass{article}
\usepackage{hyperref}
%\usepackage{exercice}
\usepackage{proof}
\newtheorem{lemma}{Lemma}
\newtheorem{theorem}{Theorem}
\def \WhizzyTeX{Whizzy\TeX}
\begin{document}
\author{Didier R{\'e}my}
\title {A proof environment, as an exercise\\
together with its {\WhizzyTeX} companion}
\maketitle
This is a short, self-documnented article with lemmas and proofs.
Please read the source under {\WhizzyTeX} and the output
at the same time.
Just move the cursor down the file in and out of proofs environment.
You may also see the self-documented article \texttt{exercise.tex}
from the distribution.
\begin{verbatim}
\newtheorem{lemma}{Lemma}
\newtheorem{theorem}{Theorem}
\end{verbatim}
\begin{lemma}
This is the first lemma.
\end{lemma}
Some short comment on the lemma.
\begin{proof}{}%inline
This is the proof of the previous lemma.
That is the first one.
Its proof is not too long.
It just first on a few lines.
And it ends there.
\end{proof}
The text continues after the proof.
\begin{lemma}
This is the second lemma.
\end{lemma}
Some short comment on the lemma.
\begin{proof}{}%inline
This is the proof of the previous lemma.
A very long proof\ldots
\def \p{.\par}\def \pv{\p\p\p\p\p}\def \px{\pv\pv}
\px\px\px\px
That ends here.
\end{proof}
The text continues after the proof.
\begin{theorem}
This is the second lemma.
\end{theorem}
Its proof is a bit shorter.
\begin{proof}{}%inline
This is the proof of the previous lemma.
This is the proof of the previous lemma.
\end{proof}
The text continues after the proof.
\begin{lemma}
Another lemma, without a proof.
\end{lemma}
\begin{lemma}
Yet another lemma, with its proof.
\end{lemma}
\begin{proof}{}
It the numbering right?
\end{proof}
\newpage
\inputanswers{Proofs}
\end{document}