\begin{frame}[fragile]
  \frametitle{Examples}
  
  \begin{exampleblock}{Example: $a \vee b,\;\; a \to c,\;\; \neg d \to \neg b \;\;\vdash\;\; c \vee d$}
  \medskip{\small
  \begin{verbatim}
Theorem ex5 : (A \/ B) -> (A -> C) -> (~D -> ~B) 
              -> (C \/ D).\end{verbatim}
  \pause
  \begin{verbatim}
  Proof.
  imp_i HAoB. imp_i HAC. imp_i HDB.
  f_dis_e HAoB HA HB.
  dis_i1.
  imp_e A.
  ass HAC.
  ass HA.
  dis_i2.
  PBC HD.
  neg_e (B).
  imp_e (~D).
  ass HDB. ass HD. ass HB.
  Qed.\end{verbatim}
  \smallskip}
  \end{exampleblock}  
\end{frame}