$\forall$$T$,${\it T'}$:Type, $l$:($T$ List), $y$:${\it T'}$, $f$:(${\it T'}$$\rightarrow$$T$$\rightarrow$${\it T'}$). list\_accum($x$,$a$.$f$($x$,$a$); $y$; $l$) $\in$ ${\it T'}$