\begin{frame}[t]
  \begin{block}{}
  \begin{center}
  $\begin{array}{llcl}
  (1)&e\cdot x&\to&x\\
  (2)&I(x)\cdot x&\to&e\\
  (3)&(x\cdot y)\cdot z &\to& x\cdot (y\cdot z)\\
  (4)&I(x)\cdot (x\cdot z)&\to& z\\
  (8)&y\cdot e&\to& y\\
  (9)& I(I(y)) &\to& y\\
  (10)&I(e)&\to& e\\
  (11)&y\cdot I(y)&\to& e\\
  (12)&y\cdot (I(y) \cdot x) &\to& x\\
  (13)&x\cdot (y\cdot I(x\cdot y)) &\to& e
  \end{array}$
  \end{center}
  \end{block}
  
  \bigskip
  Overlap between rules (4) and (13) with critical pair:
  \pause
  \begin{center}
  \begin{tikzpicture}[very thick,->,node distance=7mm]
  \node (t) {$I(x)\cdot (x\cdot (y\cdot I(x\cdot y)))$};
  \node (l) [below left=of t] {$I(x)\cdot e$};
  \onslide<3->{\node (l') [node distance=12mm,right=of l] {$I(x)$};}
  \node (r) [below right=of t] {$y\cdot I(x\cdot y)$};
  \draw (t) -- (l) node [midway,above left] {$13$};
  \draw (t) -- (r) node [midway,above right] {$4$};
  \onslide<3->{\draw (l) -- (l') node [midway,above] {$8$};}
  \onslide<4->{\draw [dashed] (r) -- (l') node [midway,above] {$14$};}
  \end{tikzpicture}
  \end{center}

  \vspace{-1ex}
  \pause\pause
  \begin{block}{}
  (14)\hspace{2em} $y\cdot I(x\cdot y)\to I(x)$
  \end{block}
 \end{frame}