@classmethod
def remove(cls, group, function_):
    """ Remove an existing function

        If your application inherits some unwanted options from a specific
        function, you can unlink this function::

            def foo(opt, default):
                pass

            Configuration.add('create-db', function_=foo)
            Configuration.remove('create-db', function_=foo)

        :param group: group is a set of parser option
        :param function_: function to add
        """
    if group in cls.groups:
        if function_ in cls.groups[group]:
            cls.groups[group].remove(function_)
