\begin{frame}[t]
  \begin{block}{}
  \begin{center}
  $\begin{array}{llcl}
  (1)&e\cdot x&\to&x\\
  (2)&I(x)\cdot x&\to&e\\
  (3)&(x\cdot y)\cdot z &\to& x\cdot (y\cdot z)\\
  (4)&I(x)\cdot (x\cdot z)&\to& z\\
  (8)&y\cdot e&\to& y\\
  (9)& I(I(y)) &\to& y\\
  (10)&I(e)&\to& e\\
  (11)&y\cdot I(y)&\to& e\\
  (12)&y\cdot (I(y) \cdot x) &\to& x
  \end{array}$
  \end{center}
  \end{block}
  
  \bigskip
  Another overlap between rules (11) and (3) with critical pair:
  \pause
  \begin{center}
  \begin{tikzpicture}[very thick,->,node distance=7mm]
  \node (t) {$(x\cdot y)\cdot I(x\cdot y)$};
  \node (l) [below left=of t] {$x\cdot (y\cdot I(x\cdot y))$};
  \node (r) [below right=of t] {$e$};
  \draw (t) -- (l) node [midway,above left] {$3$};
  \draw (t) -- (r) node [midway,above right] {$11$};
  \onslide<3->{\draw [dashed] (l) -- (r) node [midway,above] {$13$};}
  \end{tikzpicture}
  \end{center}

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