$\forall$${\it es}$:ES, $T$:Type.
\\[0ex]strong{-}subtype($T$;E) $\Rightarrow$ ($\forall$$f$:($T$$\rightarrow$$T$). ($\forall$$x$:$T$. $f$($x$) c$\leq$ $x$) $\Rightarrow$ ($\forall$$e$:$T$. $f$$\ast\ast$($e$) $\in$ $T$))