AVLtree is a small, malloc-based, in-memory index package generally
like B-trees and hash tables. The interface resembles that of the BPLUS
(B-tree) index package.

Index creation options are:

  - fixed-length binary keys OR variable-length string keys
  - unique OR duplicate keys
  - with duplicate keys:
      standard (void *) pointers for each key OR
      instance-counting (saves time and memory)

Key insert/search time is O(log N).  References:

Adelson-Velskii, G. M., and E. M. Landis.
  "An Algorithm for the Organization of Information."
  Soviet Math. Doclady 3, 1962, pp. 1259-1263.
Knuth, D. E.
  The Art of Computer Programming, Volume 3: Sorting and Searching
  (2nd printing).  Addison-Wesley, 1975, pp. 451-468.

AVLtree was written by Gregory Tseytin, tseyting@acm.org.