[Ann] Datastructs v0.21

L.S.,

I've released 'datastructs'  version 0.21, which
provides a couple of datastructures for mzscheme.

Changes: Added avl-min, avl-max, avl-empty? and avl-atomic to the
AVL Tree implementation.

More information can be found at:

http://www.elemental-programming.or...structures.html

Best regards,

Hans Oesterholt