\begin{frame}{Turing Machine Configuration}
  So configurations are denoted by words from $\Gamma^* \times Q \times \Gamma^*$.
  \begin{exampleblock}{}
    For instance, the configuration
    \begin{center}
      \def\cellwidth{10mm}
      \def\cellheight{6mm}
      \def\cellfrom{-3}
      \def\cellto{3}
      \begin{tikzpicture}[default,-,thin]
        \draw [draw=none,fill=orange!15] (\cellfrom*\cellwidth - \cellwidth,\cellheight/2) rectangle (\cellto*\cellwidth + \cellwidth,-\cellheight/2);
        
        \foreach \i in {\cellfrom,...,\cellto} {
          \draw ({\cellwidth*(\i-0.5)},\cellheight/2) -- ({\cellwidth*(\i-0.5)},-\cellheight/2);
          \node (cell\i) at ({\cellwidth*\i},0) {};
        }
        \draw ({\cellwidth*(\cellto+0.5)},\cellheight/2) -- ({\cellwidth*(\cellto+0.5)},-\cellheight/2);
  
        \draw (\cellfrom*\cellwidth - \cellwidth,\cellheight/2) -- (\cellto*\cellwidth + \cellwidth,\cellheight/2);
        \draw (\cellfrom*\cellwidth - \cellwidth,-\cellheight/2) -- (\cellto*\cellwidth + \cellwidth,-\cellheight/2);
  
        \node [anchor=east] at (\cellfrom*\cellwidth - \cellwidth,0) {$\dots$};
        \node [anchor=west] at (\cellto*\cellwidth + \cellwidth,0) {$\dots$};
  
        \node (cu) [rectangle,rounded corners=2mm,draw,inner sep=2mm,fill=orange!15] at ($(cell0) + (0,2*\cellheight)$) {$q$};
        \draw [<->] (cu) to ($(cell0) + (0,\cellheight/2)$);
        
        
        \foreach \i/\t in {-3/\Box,-2/e,-1/d,0/a,1/b,2/b,3/\Box} {
          \node [scale=0.9] at (cell\i) {$\t$};
        }
      \end{tikzpicture}\vspace{-1ex}
    \end{center}
    can be denoted by
    \begin{talign}
      ed\,q\,abb
    \end{talign}
    \pause
    The words 
    \begin{talign}
      ed\,q\,abb\Box &&\mpause[1]{\approx}&&
      \Box ed\,q\,abb &&\mpause[1]{\approx}&&
      \Box\Box ed\,q\,abb \Box && \cdots
    \end{talign}
    denote the same configuration.
  \end{exampleblock}
  \pause
  
  \begin{goal}{}
    We write $w \approx v$ if $w$ and $v$ denote the same configuration.
  \end{goal}
\end{frame}