Name:
  Facade

Intent:
  provide a uniform, consistent interface to a set of related object

Structure:
  - create a single object (the Facade) that holds references to the related object
  - access to the Facade methods/services is uniform
  - the Facade passes requests to the related objects and returns results to the client

Notes:
  similar to Proxy and Adapater (delegation, this layer)

Notes (on binary tree)
  - implementation of Composite with NullObject
  - inserting a key in a Leaf produces something that is not a Leaf
  - the Branch constructor cannot enforce efficiently the "searchness" of the tree
  - the Facade solve all these problems