Wumpus

Copperbox revision 932.

I've added an example that throws an error when rendering due to it manipulating an empty picture. I'd known the empty picture was going to be a problem, but I've needed to construct enough functionality before feeling able to look at it. I think I'm going to need the picture tree type to be a non-empty tree. This might well simplify my bounding box problems but it won't be pain free; the list concatenation operations on the Picture language (stack, hcat, vcat) depend on the notion of an empty Picture. If I get rid of the empty Picture these operations will have to throw an error themselves on the empty list.