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