Processes export \ABbroadcast$(m)$ and $m = \ABdeliver()$. We adopt the standard Atomic Broadcast specification of~\cite{Defago2004}. \ARB requires the following properties:
\begin{itemize}[leftmargin=*]
  \item \textbf{Total Order}: 
  \begin{equation*}
    \forall m_1,m_2,\ \forall p_i,p_j:\ \ (m_1 = \ABdeliver_i()) \prec (m_2 = \ABdeliver_i()) \Rightarrow (m_1 = \ABdeliver_j()) \prec (m_2 = \ABdeliver_j())
  \end{equation*}
  \item \textbf{Integrity}: Every message delivered was previously broadcast. $\forall p_i:\ m = \ABdeliver_i() \Rightarrow \exists p_j:\ \ABbroadcast_j(m)$.
  \item \textbf{No-duplicates}: No message is delivered more than once at any process.
  \item \textbf{Validity}: If a correct process broadcasts $m$, every correct process eventually delivers $m$.
\end{itemize}