Defines the requirements that a Sublayer_type of class Range_tree_d or Segment_tree_d has to fulfill.

First of all, the class has to be derived from the abstract base class Tree_base and therefore has to provide methods make_tree, window_query, enclosing_query and is_inside with the same parameter types as the instantiated class Range_tree_d or Segment_tree_d, respectively. Furthermore a method bool is_anchor() has to be provided. If the Sublayer_type class builds a recursion anchor for class Segment_tree_d, this function is expected to return true, false otherwise.

Such a recursion anchor class is provided by the class class. Tree_anchor<Data, Window>.