Processes export \ABbroadcast$(m)$ and $m = \ABdeliver()$. \ARB requires 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*}
plus Integrity/No-duplicates/Validity (inherited from \RB and the construction).