wumpus-tree

Copperbox revision 2367.

I've re-instated monadic drawing which allows graph links as well as tree links. The new monadic drawing is run inside a TraceDrawing, so it is more compatible with other drawing objects than the previous attempt, which itself was a TraceDrawing (and hence could only be composed with the picture operators). The new type cannot be a Graphic, so it is a special case, but it is equivalent.

A caveat - the  new code is very hacky. Realizing that I needed to thread the DrawingContext without "forking" it took a lot of working out. I will clean it up tomorrow.