Wumpus

Copperbox revision 933.

I've simplified the Picture type so that it is non-empty. I thought this would make difficulties for the Picture Language (i.e. the module of picture composition operators), but this hasn't been the case.

The 'multiple' operations that layout series of pictures have been changed from [a] -> a to [a] -> a -> a. The simplest to explain is stack which overlays pictures on top of each other. Stack is now considered stackOnto and stacks a list of pictures onto a 'background' picture. By always having a background it doesn't matter if the list of pictures stacked onto it is empty - we will always have a picture (never an empty picture - which we can't construct).

This should simplify the obligations on bounding box as it never need represent the bounds of an empty picture - changing this is the next step.

I've also simplified the AffineTrans module to use a single type family declaration for the unit type of transformable things. Previously each class had its own associated type for the unit.