$\forall$$i$, $j$:$\mathbb{Z}$, $F$:(\{$i$..$j$$^{-}$\}$\rightarrow\mathbb{P}$\{u\}). ($\forall$$k$:\{$i$..$j$$^{-}$\}. Dec($F$($k$))) $\Rightarrow$ Dec($\forall$$k$:\{$i$..$j$$^{-}$\}. $F$($k$))