\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}