Changed description from

Several implementations of binary search trees (BST) and balanced binary search trees (AVL).
Verify at compile time if the operations over BST/AVL trees preserves the order of the keys
and the balance in the heights.
For both of them there's one unsafe implementation and three type-safe implementations.
The last ones differ one how the structural invariants are handled at the type level.

Several implementations of type-safe binary search trees (BST) and balanced binary search trees (AVL).

These differ on how the structural invariants are implemented at the type level.

Each of them have their own advantages and disadvantages.

This library shows different ways of implementing invariants at the type level,
each of them providing different features, all of them enforced at compile time:

  - Data invariant verification: assert at compile time if any given tree is BST/AVL.

  - Program certification: verify at compile time if the implementation of the operations
    over BST/AVL trees preserves the order of the keys and the balance in the heights.

  - Type-safe data constructors: implementation of tree constructors that throw an error at compile time
    if the tree being constructed is not BST/AVL.