\begin{tabbing}
strong{-}subtype($A$; $B$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=subtype\_rel($A$; $B$)\+
\\[0ex]c$\wedge$ subtype\_rel(\{$b$:$B$$\mid$ $\exists$$a$:$A$. ($b$ = $a$)\} ; $A$)
\\[0ex]c$\wedge$ ($\forall$$a_{1}$,$a_{2}$:$A$. ($a_{1}$ = $a_{2}$) $\Rightarrow$ ($a_{1}$ = $a_{2}$))
\-
\end{tabbing}