$\forall$${\it es}$:event\_system\{i:l\}, $e$,${\it e'}$:es{-}E(${\it es}$).
\\[0ex]es{-}locl(${\it es}$; $e$; ${\it e'}$) $\Rightarrow$ sqequal([$e$, ${\it e'}$]; append([$e$, es{-}pred(${\it es}$; ${\it e'}$)]; cons(${\it e'}$; [])))