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.