basic
\\[0ex]Ident($T$;${\it op}$;${\it id}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$:$T$. ($x$ ${\it op}$ ${\it id}$) = $x$ $\in$ $T$ \& (${\it id}$ ${\it op}$ $x$) = $x$ $\in$ $T$