strong{-}subtype($A$;$B$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$A$ $\subseteq\rho$ $B$ \& \{$b$:$B$$\mid$ $\exists$$a$:$A$. $b$ $=$ $a$ $\in$ $B$ \} $\subseteq\rho$ $A$ \& ($\forall$$a_{1}$:$A$, $a_{2}$:$A$. $a_{1}$ $=$ $a_{2}$ $\in$ $B$ $\Rightarrow$ $a_{1}$ $=$ $a_{2}$ $\in$ $A$)