\begin{frame}[t]
  \begin{block}{} 
  \begin{center}
  $\begin{array}{llcl}
  (1)&e\cdot x&\to&x\\
  (2)&I(x)\cdot x&\to&e\\
  (3)&(x\cdot y)\cdot z &\to& x\cdot (y\cdot z)
  \end{array}$
  \end{center}
  \end{block}
    
  \bigskip\pause
  Critical pairs:
  \begin{itemize}
  \onslide<3->
    \item between (1) and (3): $\langle y\cdot z,\; e\cdot (y\cdot z)\rangle$ \hfill\onslide<4->{\alert<4>{convergent\quad\quad}}
  \onslide<5->
    \item between (2) and (3): $\langle e\cdot z,\; I(x)\cdot (x\cdot z)\rangle$ \hfill\onslide<6->{\alert<6>{not convergent\quad\quad}}
  \end{itemize} 
  \onslide<7->
  
  \begin{center}
  \begin{tikzpicture}[very thick,->]
  \node (t) {$(I(x)\cdot x)\cdot z$};
  \node (l) [below left=of t] {$e\cdot z$};
  \node (r) [below right=of t] {$I(x)\cdot(x\cdot z)$};
  \draw (t) -- (l) node [midway,above left] {$2$};
  \draw (t) -- (r) node [midway,above right] {$3$};
  \onslide<8->{\node (l') [node distance=7mm,below=of l] {$z$};}
  \onslide<8->{\draw (l) -- (l') node [midway,left] {$1$};}
  \onslide<9->{\draw [dashed] (r) -- (l') node [midway,above right] {$4$};}
  \end{tikzpicture}
  \end{center}
  
  \vspace{-1ex}
  \onslide<9->
  Add a rule: 
  \begin{block}{}
  (4)\hspace{2em} $I(x)\cdot (x\cdot z)\to  z$
  \end{block}
\end{frame}