$\forall$${\it es}$:ES, ${\it ff}$:FIFO. ${\it ff}$.Receiver $\in$ $j$,$i$:${\it ff}$.C$\rightarrow$\{$e$:E$\mid$ ${\it ff}$.S($j$,$i$,$e$)\} $\rightarrow$\{$e$:E$\mid$ ${\it ff}$.R($i$,$e$)\}