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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _extern_decl(return_type, arg_types):
 """A decorator for methods corresponding to extern functions. All types should be strings. The _FFISpecification class is able to automatically convert these into method declarations for cffi. """

  def wrapper(func):
    signature = _ExternSignature(
      return_type=str(return_type),
      method_name=str(func.__name__),
      arg_types=tuple(arg_types))
    func.extern_signature = signature
    return func
  return wrapper