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.
  
  from eLisp.model import Symbol, Pair, EmptyList
  from eLisp.expr.util import (
          is_tagged_list, pair_to_list, car, cdr, cons, cadr, caddr, cadddr)
  from eLisp.builtin import BUILTIN_PROCEDURES
  
  def is_application(expr):
      return isinstance(expr, Pair)
  
  def operator(expr):
      return car(expr)
  
  def operands(expr):
      return cdr(expr)
  
  def no_operands(ops):
      return ops is EmptyList
  
  def first_operand(ops):
      return car(ops)
  
  def rest_operands(ops):
      return cdr(ops)
  
  def make_procedure(params, body, env):
      return cons(Symbol('procedure'), 
              cons(params, cons(body, cons(env, EmptyList))))
  
  def is_compound_procedure(expr):
      return is_tagged_list(expr, Symbol('procedure'))
  
  def procedure_parameters(expr):
      return cadr(expr)
  
  def procedure_body(expr):
      return caddr(expr)
  
  def procedure_environment(expr):
      return cadddr(expr)
  
  def get_procedure_repr(expr):
      return '#<procedure %s %s <procedure-env>' % (
              procedure_parameters(expr), procedure_body(expr))
  
  def is_primitive_procedure(expr):
      return is_tagged_list(expr, Symbol('primitive'))
  
  def primitive_implementation(expr):
      return [name for name, _ in BUILTIN_PROCEDURES]
  
  def primitive_procedure_values():
      return [Pair(Symbol('primitive'), Pair(proc, EmptyList)) 
              for _, proc in BUILTIN_PROCEDURES]
  
  def apply_primitive_procedure(proc, args):
      func = primitive_implementation(proc)
      return func(*pair_to_list(args))
  
subject: add procedure expression