def modify(self, func):<EOL>
def modifier(state):<EOL><INDENT>return self._optic.over(state, func)<EOL><DEDENT>return modifier<EOL>
Apply a function to the focus.

            >>> from lenses import lens
            >>> convert_item_one_to_string = lens[1].modify(str)
            >>> convert_item_one_to_string([1, 2, 3])
            [1, '2', 3]
            >>> add_ten_to_item_one = lens[1].modify(lambda n: n + 10)
            >>> add_ten_to_item_one([1, 2, 3])
            [1, 12, 3]