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.
  
  class BaseType(object):
      def __init__(self, val):
          self.val = val
  
      def __str__(self):
          pass
  
      def __eq__(self, other):
          return isinstance(other, self.__class__) and self.val == other.val
  
      def __lt__(self, other):
          return isinstance(other, self.__class__) and self.val < other.val
  
      def __ne__(self, other):
          return not self.__eq__(other)
  
      def __le__(self, other):
          return self.__lt__(other) or self.__eq__(other)
  
      def __gt__(self, other):
          return not self.__le__(other)
  
      def __ge__(self, other):
          return not self.__lt__(other)
  
      def __nonzero__(self):
          return True
  
  class Number(BaseType):
      def __str__(self):
          return str(self.value)
  
  class Boolean(BaseType):
      def __str__(self):
          return '#t' if self.value else '#f'
  
      def __nonzero__(self):
          return self.value
  
  class Character(BaseType):
      def __str__(self):
          return '#\\%s' % str(self.value)
  
  class String(BaseType):
      def __str__(self):
          return '"%s"' % str(self.value)
  
  class Symbol(object):
      def __init__(self, name):
          self.name = name
  
      def __str__(self):
          return self.name
  
      def __eq__(self, other):
          return isinstance(other, Symbol) and self.name == other.name
  
      def __nonzero__(self):
          return True
  
  class EmptyList(object):
      def __str__(self):
          return '()'
  
      def __eq__(self, other):
          return isinstance(other, self.__class__)
  
      def __nonzero__(self):
          return True
  
  EmptyList = EmptyList()
  
  class Pair(object):
      def __init__(self, head, tail):
          self.head = head
          self.tail = tail
  
      def __str__(self):
          return '(%s)' % self._write_pair(self)
  
      @staticmethod
      def _write_pair(pair):
          head, tail = pair.head, pair.tail
  
          output = str(head)
          if isinstance(tail, pair):
              output += ' %s' % Pair._write_pair(tail)
              return output
  
          if tail is EmptyList:
              return output
  
          output += ' . %s' % str(tail)
          return output
  
subject: add base model