\begin{tabbing}
basic
\\[0ex]BiLinear($T$;${\it pl}$;${\it tm}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=$\forall$$a$:$T$.\+
\\[0ex]$\forall$$x$:$T$, $y$:$T$.
\\[0ex]($a$ ${\it tm}$ ($x$ ${\it pl}$ $y$)) = (($a$ ${\it tm}$ $x$) ${\it pl}$ ($a$ ${\it tm}$ $y$)) $\in$ $T$
\\[0ex]\& (($x$ ${\it pl}$ $y$) ${\it tm}$ $a$) = (($x$ ${\it tm}$ $a$) ${\it pl}$ ($y$ ${\it tm}$ $a$)) $\in$ $T$
\-
\end{tabbing}