anchor.diff:        }
         return result;
       };
  +    _natives['true'] = true;
  +    _natives['false'] = false;
       _natives['and'] = (Environment env, Dynamic args) {
         while (args != null) {
           if (!eval(env, args.head)) {
  
positive.code:         var inner = lambda_env.create();
          var names = lambda_args.head;
          var values = evalArgs(env, args);
          while (names is Cons && values is Cons) {
            inner[names.head] = values.head;
            names = names.tail;
            values = values.tail;
          }
          var result = null;
          var stmt = lambda_args.tail;
          while (stmt is Cons) {
            result = eval(inner, stmt.head);
            stmt = stmt.tail;
          }
  
negative.code: // Copyright (c) 2012, Lukas Renggli <renggli@gmail.com>
  
  /** Collection of native functions. */
  class Natives {
  
    static Map<String, Dynamic> _natives;
  
    /** Looks a native function up. */
    static Dynamic find(String name) {
      _initialize();
      return _natives[name];
    }
    
    /** Imports all the native functions into the [environment]. */
    static void importAllInto(Environment env) {
      _initialize();
      _natives.forEach((key, value) {
        env[new Symbol(key)] = value;
      });
    }
  
    static void _initialize() {
      if (_natives == null) {
        _natives = new Map();
        _basicFunctions();
        _controlStructures();
        _arithmeticMethods();
        _arithmeticComparators();
        _listOperators();
      }
    }
  
    static void _basicFunctions() {
  