2011-11-4: Update by Ondrej Kuncar
  - Changes in the monolithic version of formalization:
  - Added formalization of deletion
  - The rest (namely formalization of insertion)  was rewritten to Isar => speedup
  - bal_l and bal_r were replaced by more convenient mkt_bal_l and mkt_bal_r functions
  - insrt renamed to insert

2006-05-25: Update by Tobias Nipkow
  - Did AVL trees properly, with height information in each node.
  - Completely revised code and proofs
  - Two developments, a monolithic and an incremental one.
  - Got rid of the "extended tree type" by Brucker et al
    because it stored the wrong information (bal!) and nothing useful
    was proved about it.

2004-06-09: Update by Achim Brucker, Burkhart Wolff and Jan Smaus
  - parameterized the tree definitions
  - added a "efficient" variant of is-in (is_in_eff)
  - some proofs about is_ord and is_in_eff
  - a extended tree type that stores balancing information 
    in its node and some proofs about it 

2004-03-19: First submission