anchor.diff:  %%% Notes:
   %%% - LETREC is implemented via finite unfolding of a local
   %%%   non-recursive environment fragment
  +%%% - Evaluation is a two-step process: a top-level S-expression is
  +%%%   first parsed and converted to an abstract syntax tree (AST),
  +%%%   which is then interpreted.  Function closures record their
  +%%%   bodies as ASTs, not as S-expressions.
   
   -module(es_eval).
   
  
positive.code:     _ -> false
    end.
  
  %%
  
  env_empty() -> gb_trees:empty().
  env_lookup(Env, Var) -> gb_trees:lookup(Var, Env).
  env_enter(Env, Var, Val) -> gb_trees:enter(Var, Val, Env).
  
  
negative.code: %%% -*- erlang-indent-level: 2 -*-
  %%%
  %%%   Copyright 2014-2015 Mikael Pettersson
  %%%
  %%%   Licensed under the Apache License, Version 2.0 (the "License");
  %%%   you may not use this file except in compliance with the License.
  