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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 env_resolver(cls, var_name: str, env_name: str = None, default: Any = _NONE) -> 'Configuration': """ Method for configuring environment resolver. :param var_name: Variable name :param env_name: An optional environment variable name. If not set\ haps looks for `HAPS_var_name` :param default: Default value for variable. If it's a callable,\ is called before return. If not provided\ :class:`~haps.exceptions.UnknownConfigVariable` is raised :return: :class:`~haps.config.Configuration` instance for easy\ chaining """


        cls.resolver(var_name)(
            partial(
                _env_resolver, var_name=var_name, env_name=env_name,
                default=default))
        return cls()