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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 _build_doc(func_name, desc, arg_names, arg_types, arg_desc, key_var_num_args=None, ret_type=None):
 """Build docstring for symbolic functions."""

    param_str = _build_param_doc(arg_names, arg_types, arg_desc)
    if key_var_num_args:
        desc += '\nThis function support variable length of positional input.'
    doc_str = ('%s\n\n' +
               '%s\n' +
               'name : string, optional.\n' +
               '    Name of the resulting symbol.\n\n' +
               'Returns\n' +
               '-------\n' +
               'Symbol\n' +
               '    The result symbol.')
    doc_str = doc_str % (desc, param_str)
    extra_doc = "\n" + '\n'.join([x.__doc__ for x in type.__subclasses__(SymbolDoc)
                                  if x.__name__ == '%sDoc' % func_name])
    doc_str += _re.sub(_re.compile("    "), "", extra_doc)
    doc_str = _re.sub('NDArray-or-Symbol', 'Symbol', doc_str)
    return doc_str