\begin{frame}[t]
  \begin{block}{}
  \begin{center}
  $\begin{array}{llcl}
  (1)&e\cdot x&\to&x\\
  (2)&I(x)\cdot x&\to&e\\
  (3)&(x\cdot y)\cdot z &\to& x\cdot (y\cdot z)\\
  (4)&I(x)\cdot (x\cdot z)&\to& z\\
  (5)&I(e)\cdot z&\to& z\\
  (6)& I(I(y))\cdot e &\to& y\\
  (7)& I(I(y))\cdot x &\to&  y\cdot x\\
  (8)&y\cdot e&\to& y
  \end{array}$
  \end{center}
  \end{block}
  
  \bigskip
  Overlap between rules (8) and (6) with critical pair:
  \pause
  \begin{center}
  \begin{tikzpicture}[very thick,->]
  \node (t) {$I(I(y))\cdot e$};
  \node (l) [below left=of t] {$I(I(y))$};
  \node (r) [below right=of t] {$y$};
  \draw (t) -- (l) node [midway,above left] {$8$};
  \draw (t) -- (r) node [midway,above right] {$6$};
  \onslide<3->{\draw [dashed] (l) -- (r) node [midway,above] {$9$};}
  \end{tikzpicture}
  \end{center}

  \pause
  Add a rule: 
  \begin{block}{}
  (9)\hspace{2em} $I(I(y))\to  y$
  \end{block}
\end{frame}