causal\_order($L$;$R$;$P$;$Q$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$i$:\{0..$\parallel$$L$$\parallel^{-}$\}. $Q$($i$) $\Rightarrow$ ($\exists$$j$:\{0..$\parallel$$L$$\parallel^{-}$\}. $j$$\leq$$i$ \& $P$($j$) \& $R$($j$,$i$))