\begin{frame}[t]
  \begin{block}{}
  \begin{center}
  $\begin{array}{llcl}
  (1)&e\cdot x&\to&x\\
  (2)&I(x)\cdot x&\to&e\\
  (3)&(x\cdot y)\cdot z &\to& x\cdot (y\cdot z)\\
  (4)&I(x)\cdot (x\cdot z)&\to& z
  \end{array}$
  \end{center}
  \end{block}
    
  \bigskip\pause
  Overlap of the rules (1) and (4), (2) and (4), (3) and (4), (4) and (4).
  
  \medskip\pause
  We start with (4) and (1) with critical pair:
  $\langle I(e)\cdot z,\; z\rangle$
  
  \bigskip
  \begin{center}
  \begin{tikzpicture}[very thick,->]
  \node (t) {$I(e)\cdot (e\cdot z)$};
  \node (l) [below left=of t] {$I(e)\cdot z$};
  \node (r) [below right=of t] {$z$};
  \draw (t) -- (l) node [midway,above left] {$1$};
  \draw (t) -- (r) node [midway,above right] {$4$};
  \onslide<4->{\draw [dashed] (l) -- (r) node [midway,above] {$5$};}
  \end{tikzpicture}
  \end{center}
  
  \pause
  Add a rule: 
  \begin{block}{}
  (5)\hspace{2em} $I(e)\cdot z\to  z$
  \end{block}
\end{frame}