\begin{tabbing}
$\forall$$A$,$B$:(top List), $y$,$f$:top.
\\[0ex]sqequal(\=list\_accum($x$,$a$.$f$($x$,$a$); $y$; append($A$; $B$));\+
\\[0ex]list\_accum($x$,$a$.$f$($x$,$a$); list\_accum($x$,$a$.$f$($x$,$a$); $y$; $A$); $B$))
\-
\end{tabbing}