\begin{frame}{Exercise}
  \begin{exampleblock}{}
    Consider the following grammar
    \begin{talign}
      S &\to ABaC 
    & A &\to BC
    & B &\to b \mid \lambda
    & D &\to d \\
    &&&& C &\to D \mid \lambda
    \end{talign}
    \pause    
    What variables are erasable?
    \begin{itemize}
    \pause
      \item $A$, $B$ and $C$
    \end{itemize}
    \pause\medskip

    Construct the resulting grammar after removing all $\lambda$-rules:
    \pause
    \begin{talign}
    S &\to ABaC \mpause[1]{\mid BaC} \mpause{\mid AaC} \mpause{\mid ABa} \mpause{\mid aC} \mpause{\mid Ba} \mpause{\mid Aa} \mpause{\mid a} \\
    A &\to BC \mpause{\mid C} \mpause{\mid B} \mpause{\onslide<-15>{\mid \lambda}} \\
    B &\to b \onslide<-15>{\mid \lambda} \hspace{2cm}
    C \to D \onslide<-15>{\mid \lambda} \hspace{2cm}
    D \to d\mpause{}
    \end{talign}
  \end{exampleblock}
\end{frame}

\themex{  Unit Production Rules}