> module Pic where

> import Picture

> data Pic s = EmptyPic
>            | Above (Pic s) (Pic s)
>            | Prim s
>            | Prop Property (Pic s)

> instance HasVert s => HasVert (Pic s) where
>   vertMap f EmptyPic = EmptyPic
>   vertMap f (Prim s) = Prim (vertMap f s)
>   vertMap f (Prop pr p) = Prop pr (vertMap f p)
>   vertMap f (Above p q) = Above (vertMap f p) (vertMap f q)

>   bbox EmptyPic = Nothing
>   bbox (Prim s) = bbox s
>   bbox (Prop _ p) = bbox p
>   bbox (Above p q) = case bbox p of
>     Nothing -> bbox q
>     b1@(Just (p1,p2)) -> case bbox q of
>       Nothing -> b1
>       Just (p3,p4) -> Just $ vertList_bb [p1,p2,p3,p4]

> toOrigin p = case bbox p of
>                Nothing -> p
>                Just (ll,ru) -> vertMap (^-^ ll) p

> instance Picture Pic where
>   emptyPic = EmptyPic
>   above    = Above
>   prim     = Prim
>   setProp  = Prop