The purpose of this repository is to provide a collection of
implementations of trees.

In contrast to existing libraries such as cl-containers, we do not
want to impose a particular use for the trees.  Instead, we aim for a
stratified design, allowing client code to choose between different
levels of abstraction.

As a consequence of this policy, we provide low-level interfaces where
the concrete representation is exposed, but also high level interfaces
where the trees can be used as search trees or as trees that represent
sequences of objects.