\begin{frame}[fragile,t]{Programs {\tt Fac1}, {\tt Fac2}}
  The \emph{factorial} $\forestgreen{n}$ of a natural number $\forestgreen{n}$ is defined inductively by:  
  \begin{talign}
    \fac{0} &\defdby 1 \\
    \fac{(\forestgreen{n}+1)} &\defdby (\forestgreen{n}+1) \cdot \fac{\forestgreen{n}}
  \end{talign}
    
  \begin{center}
    \begin{minipage}{0.45\textwidth}
      \begin{exampleblock}{Program {\tt Fac1}}
      \begin{lstlisting}
    y = 1;
    z = 0;
    while (z != x) {
      z = z + 1;
      y = y * z;
    }
      \end{lstlisting}  
      \end{exampleblock}      
    \end{minipage}
    \pause\hspace*{3ex}
    \begin{minipage}{0.45\textwidth}
      \begin{exampleblock}{Program {\tt Fac2}}
      \begin{lstlisting}
    y = 1;
    while (x != 0) {
      y = x * y;
      x = x - 1;
    }
      \end{lstlisting}  
      \end{exampleblock}      
    \end{minipage}
  \end{center}  
\end{frame}