bb43febb ​

input = fill 10v (fill 10h black) |> fold (λr. rect (r, grey)) #0
output = fill 10v (fill 10h black) |> fold (λr. rect (r, grey)) #0 |> fold (λr. rect ((r.1 + 1, r.2 + 1, r.3 - 1, r.4 - 1), red)) #0
(UList (VInt, HInt, VInt, HInt))
({(3v, 1h, 7v, 5h), (4v, 7h, 7v, 9h)})
({(8v, 5h, 10v, 9h), (2v, 2h, 6v, 7h)})
({(5v, 4h, 10v, 10h), (1v, 1h, 3v, 6h)})