ma{-}abs{-}interface(${\it es}$;$X$)($e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$if ma{-}in{-}interface(${\it es}$;$X$;$e$) then ma{-}interface{-}val(${\it es}$;$X$;$e$) else inr $\cdot$  fi