\begin{frame}
  \frametitle{Example: ATM Transaction}
  
  To comply with the 2PL, the ATM transaction 
  \alert{must not acquire new locks after a lock has been released}.
  \bigskip
  
  
  \begin{exampleblock}{ATM withdrawal with 2 Phase Locking}
    \nr{1} \textcolor{dgreen}{$\fun{xlock}(\var{account})$}\\
    \nr{2} $\var{balance} \set \fun{read}(\var{account})$\\
    \nr{3} $\var{balance} \set \var{balance}-100$\\
    \nr{4} $\fun{write}(\var{account},\var{balance})$\\
    \nr{5} \textcolor{dgreen}{$\fun{unlock}(\var{account})$} 
  \end{exampleblock}
\end{frame}