0b17323b ​

input = fill 15v (fill 15h black) |> fold (λr. rect ((r.1, r.2, r.1, r.2), blue)) #0
output = fill 15v (fill 15h black) |> fold (λr. rect ((r.1, r.2, r.1, r.2), red)) #1 |> fold (λr. rect ((r.1, r.2, r.1, r.2), blue)) #0
(UList (VInt, HInt), UList (VInt, HInt))
({(6v, 6h), (4v, 4h), (2v, 2h)}, {(14v, 14h), (12v, 12h), (10v, 10h), (8v, 8h)})
({(1v, 1h), (9v, 9h), (5v, 5h)}, {(13v, 13h)})
({(1v, 1h), (7v, 7h), (4v, 4h)}, ?)