Implementation of :py:func:`pynspect.traversers.RuleTreeTraverser.function` interface.

def function(self, rule, args, **kwargs):
        """
        Implementation of :py:func:`pynspect.traversers.RuleTreeTraverser.function` interface.
        """
        fname = rule.function
        try:
            return self.functions[fname](args)
        except KeyError:
            raise FilteringRuleException("Invalid function name '{}'".format(fname))