\begin{frame}{($\Rightarrow$) From NFAs to Regular Expressions (2)}
  \begin{block}{}%[$\Rightarrow$ continued]
    \emph{Step 2:}
    \smallskip
    
    We remove all double arrows.
    \pause\medskip
    
    We use transition graphs with \alert{regular expressions as labels}.
    \pause\medskip

    If there are $2$ arrows from a state $q_1$ to $q_2$ with labels $r_1$ and $r_2$
    replace them by one arrow with label $r_1 + r_2$:
    \begin{center}
    \begin{tikzpicture}[default,node distance=20mm,->,s/.style={minimum size=5mm}]
      \node (q1) [state,s] {$q_1$};
      \node (q2) [state,s,right of=q1] {$q_2$}; 
  
      \draw (q1) to[bend left=15] node [above] {$r_1$} (q2);
      \draw (q1) to[bend left=-15] node [below] {$r_2$} (q2);
      
      \node at (30mm,0mm) {$\Rightarrow$};
      
      \begin{scope}[xshift=40mm]
      \node (q1) [state,s] {$q_1$};
      \node (q2) [state,s,right of=q1] {$q_2$}; 
  
      \draw (q1) to node [rectangle,above,inner sep=1mm] {$r_1 + r_2$} (q2);
      \end{scope}
    \end{tikzpicture}
    \end{center}
    \pause
    Note that $q_1$ can be equal to $q_2$.
    Then the arrows are loops!
    \pause\medskip
    
    \emph{We remove all double arrows before continuing with Step 3.}
  \end{block}
  \vspace{10cm}
\end{frame}