Example of arithmetic expression parsing, but producing a labelled
forest.

Note use of polymorphic type in the labels. See the code more more
discussion.

"make run" to run the test case.

For Hugs, load up Hugs.lhs - it is a simplified version of Main, with entry
point "test :: String -> IO ()"

NOTE: you need the -98 flag on Hugs, owing to non-standard class use