$\forall$${\it es}$:ES, ${\it ff}$:FIFO, $p$:(E$\rightarrow\mathbb{P}$), $e$:E, ${\it sndr}$, ${\it rcvr}$:${\it ff}$.C. [$e$: ${\it sndr}$ $\leftarrow$$p$$--$ ${\it rcvr}$] $\in$ $\mathbb{P}$