$\forall$${\it es}$:ES, ${\it ff}$:FIFO, ${\it is\_req}$, ${\it is\_ack}$:(E$\rightarrow\mathbb{P}$), ${\it sndr}$, ${\it rcvr}$:${\it ff}$.C, $r$, $a$:E.
\\[0ex]crossed{-}pair\{i:l\}(${\it es}$;${\it ff}$;${\it is\_req}$;${\it is\_ack}$;${\it sndr}$;${\it rcvr}$;$r$;$a$) $\in$ $\mathbb{P}$