anchor.diff:          { order :: pos_integer() % aka N
           , root :: #page{}
           }).
  +
  +-type item(T) :: {K :: T, P :: pageid()}.
  +-type page(T) :: #page{e :: {item(T)}}.
  +-type btree(T) :: #btree{root :: page(T)}.
   
   %%%_* Creating an empty B-tree =================================================
   
  
positive.code: 
  -export([check/2, print/2]). % for debugging only
  
  %%%_* Types and macros =========================================================
  
  %-define(DEBUG, true).
  
negative.code: %%% Author      : Mikael Pettersson <mikael.pettersson@klarna.com>
  %%% Description : Erlang implementation of B-tree sets
  %%%
  %%% Copyright (c) 2016-2017 Klarna AB
  %%%
  %%% This file is provided to you under the Apache License,
  %%% Version 2.0 (the "License"); you may not use this file
  