$\forall$${\it es}$:event\_system\{i:l\}, $e_{1}$,$e_{2}$:es{-}E(${\it es}$).
\\[0ex]es{-}le(${\it es}$; $e_{1}$; $e_{2}$) $\Rightarrow$ sqequal(last(es{-}hist\{i:l\}(${\it es}$;$e_{1}$;$e_{2}$)); es{-}info(${\it es}$;$e_{2}$))