new_contents: #!/usr/bin/env python
  # -*- encoding: utf-8 -*-
  #
  # Copyright (c) 2015 ASMlover. All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without
  # modification, are permitted provided that the following conditions
  # are met:
  #
  #  * Redistributions of source code must retain the above copyright
  #    notice, this list ofconditions and the following disclaimer.
  #
  #  * Redistributions in binary form must reproduce the above copyright
  #    notice, this list of conditions and the following disclaimer in
  #    the documentation and/or other materialsprovided with the
  #    distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
  # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
  # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  # POSSIBILITY OF SUCH DAMAGE.
  
  import os
  
  from eLisp.lexer import Lexer
  from eLisp.model import Pair, Symbol, EmptyList
  from eLisp.parser import Parser
  
  def cons(head, tail):
      return Pair(head, tail)
  
  def car(pair):
      return pair.head
  
  def cdr(pair):
      return pair.tail
  
  def tolist(*expr):
      def inner(args):
          if not args:
              return EmptyList
          return cons(args[0], inner(args[1:]))
      return inner(expr)
  
  caar = lambda pair: car(car(pair))
  cadr = lambda pair: car(cdr(pair))
  cddr = lambda pair: cdr(cdr(pair))
  caddr = lambda pair: car(cdr(cdr(pair)))
  cadar = lambda pair: car(cdr(car(pair)))
  cdddr = lambda pair: cdr(cdr(cdr(pair)))
  caadr = lambda pair: car(car(cdr(pair)))
  cdadr = lambda pair: cdr(car(cdr(pair)))
  cadddr = lambda pair: car(cdr(cdr(cdr(pair))))
  
  def is_tagged_list(expr, tag):
      if isinstance(expr, Pair) and car(expr) == tag:
          return True
      return False
  
  def is_symbol(expr):
      return isinstance(expr, Symbol)
  
  def pair_to_list(pair):
      result = []
      if pair is EmptyList:
          return result
  
      head, tail = pair.head, pair.tail
      result.append(head)
      while tail is not EmptyList:
          head, tail = tail.head, tail.tail
          result.append(head)
  
      return result
  
  def is_load(expr):
      return is_tagged_list(expr, Symbol('load'))
  
  def load(interpreter, expression):
      '''
      read expressions and definitions from file
      '''
      filepath = cadr(expression).val
      data = open(os.path.abspath(filepath)).read()
      parser = Parser(Lexer(data))
  
      for expr in parser.Parse():
          interpreter.interpret(expr)
  
subject: add util tool functions for expr