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\begin{frame}
\small

\begin{block}{Motivation}
\smallskip
\alert<1-6>{concrete} rewrite formalisms
\begin{itemize}
\item
string rewriting
\item<2->
term rewriting
\item<3->
graph rewriting 
\item<4->
$\lambda$-calculus
\item<5->
interaction nets
\item<6->
$\cdots$
\end{itemize}
\onslide<7->
\alert<2->{abstract} rewriting
\begin{itemize}
\item
no structure on objects that are rewritten
\item<8->
uniform presentation of properties and proofs
\end{itemize}
\end{block}

\end{frame}