<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tile(self, map_id, x, y, z, retina=False, file_format="png", style_id=None, timestamp=None):
 """Returns an image tile, vector tile, or UTFGrid in the specified file format. Parameters map_id : str The tile's unique identifier in the format username.id. x : int The tile's column, where 0 is the minimum value and ((2**z) - 1) is the maximum value. y : int The tile's row, where 0 is the minimum value and ((2**z) - 1) is the maximum value. z : int The tile's zoom level, where 0 is the minimum value and 20 is the maximum value. retina : bool, optional The tile's scale, where True indicates Retina scale (double scale) and False indicates regular scale. The default value is false. file_format : str, optional The tile's file format. The default value is png. style_id : str, optional The tile's style id. style_id must be used together with timestamp. timestamp : str, optional The style id's ISO-formatted timestamp, found by accessing the "modified" property of a style object. timestamp must be used together with style_id. Returns ------- request.Response The response object with a tile in the specified format. """


        # Check x, y, and z.

        if x is None or y is None or z is None:
            raise ValidationError(
                "x, y, and z must be not be None"
            )

        # Validate x, y, z, retina, and file_format.

        x = self._validate_x(x, z)
        y = self._validate_y(y, z)
        z = self._validate_z(z)
        retina = self._validate_retina(retina)
        file_format = self._validate_file_format(file_format)

        # Create dict to assist in building URI resource path.

        path_values = dict(
            map_id=map_id,
            x=str(x),
            y=str(y),
            z=str(z)
        )

        # Start building URI resource path.

        path_part = "/{map_id}/{z}/{x}/{y}"
        uri = URITemplate(self.base_uri + path_part).expand(**path_values)

        # Finish building URI resource path.
        # As in static.py, this two-part process avoids 
        # undesired escaping of "@" in "@2x."

        path_part = "{}.{}".format(retina, file_format)
        uri += path_part

        # Validate timestamp and build URI query parameters.
 
        query_parameters = dict()

        if style_id is not None and timestamp is not None:
            timestamp = self._validate_timestamp(timestamp)
            style = "{}@{}".format(style_id, timestamp)
            query_parameters["style"] = style

        # Send HTTP GET request.

        response = self.session.get(uri, params=query_parameters)
        self.handle_http_error(response)

        return response