Implement red black trees in duck

[girving]

See http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=445, or Okasaki, "Red-Black Trees in a Functional Setting".

[dylex]

For comparison, haskell uses size balanced binary trees (http://www.swiss.ai.mit.edu/~adams/BB/ , http://www.haskell.org/ghc/docs/latest/html/libraries/containers-0.4.0.0/Data-Map.html ).

[girving]

The Okasaki paper contains extremely simple and elegant red black tree code. Neither of your links work for me, so I'm not sure how they compare.