$\forall$${\it es}$:ES, ${\it ff}$:FIFO, ${\it f2f+}$:F2F+{-}decls, $e$:E, ${\it sndr}$, ${\it rcvr}$:${\it ff}$.C.
\\[0ex][$e$: ${\it rcvr}$ $--$is\_ack $\rightarrow$ ${\it sndr}$] $\Rightarrow$ (loc($e$) = ${\it rcvr}$ $\in$ Id)