\begin{tabbing}
$\forall$${\it es}$:ES, ${\it ff}$:FIFO, ${\it f2f+}$:F2F+{-}decls, ${\it sndr}$, ${\it rcvr}$:${\it ff}$.C, $e$, ${\it e'}$:E.
\\[0ex](f2f+{-}p+($e$,${\it e'}$))
\\[0ex]$\Rightarrow$ \=\{([$e$: ${\it sndr}$ $--$is\_req  $\rightarrow$ ${\it rcvr}$] $\vee$ [$e$: ${\it rcvr}$ $--$is\_ack $\rightarrow$ ${\it sndr}$])\+
\\[0ex]\& ([${\it e'}$: ${\it sndr}$ $--$is\_req  $\rightarrow$ ${\it rcvr}$] $\vee$ [${\it e'}$: ${\it rcvr}$ $--$is\_ack $\rightarrow$ ${\it sndr}$])\}
\-
\end{tabbing}