fpf{-}accum($z$,$a$,$v$.$f$($z$;$a$;$v$);$y$;$x$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$list\_accum($z$,$a$.$f$($z$;$a$;2of($x$)($a$));$y$;1of($x$))