def call(self, method_name, *args, **kwargs):<EOL>
caller = operator.methodcaller(method_name, *args, **kwargs)<EOL>return self.modify(caller)<EOL>
Call a method on the focus. The method must return a new
        value for the focus.

            >>> from lenses import lens
            >>> lens[2].call('upper')(['alpha', 'beta', 'gamma'])
            ['alpha', 'beta', 'GAMMA']

        As a shortcut, you can include the name of the method you want
        to call immediately after `call_`:

            >>> lens[2].call_upper()(['alpha', 'beta', 'gamma'])
            ['alpha', 'beta', 'GAMMA']