actually replace function

def add_function(self,
                     fun=None,
                     name=None,
                     fun_type=FUN_TYPE):
        """actually replace function"""
        if not name:
            if six.PY2:
                name = fun.func_name
            else:
                name = fun.__name__

        self.say('adding fun(%s)' % name, verbosity=50)
        self.say('adding fun_type:%s' % fun_type, verbosity=50)

        if self.function_exists(name):
            self.cry('overwriting :fun(%s)' % name, verbosity=10)

        self.say('added :' + name, verbosity=10)
        self._rule_functions[name] = RuleFunction(name, fun, fun_type)
        return True