$\forall$${\it es}$:ES, ${\it ff}$:FIFO, ${\it f2f+}$:F2F+{-}decls. S\_dcdr $\in$ $i$,$j$:${\it ff}$.C$\rightarrow$$e$:E$\rightarrow$Dec(${\it ff}$.S($i$,$j$,$e$))