\begin{tabbing}
fpf{-}join(${\it eq}$; $f$; $g$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=$<$append(($f$.1); filter(($\lambda$$a$.$\neg_{b}$fpf{-}dom(${\it eq}$; $a$; $f$)); ($g$.1)))\+
\\[0ex], $\lambda$$a$.fpf{-}cap($f$; ${\it eq}$; $a$; fpf{-}ap($g$; ${\it eq}$; $a$))
\\[0ex]$>$
\-
\end{tabbing}