I think that the line below will do what you want, but I haven't had a chance to try it, so caveat emptor. The Garbage field returns a SrgsRuleRef with the attribute you want. Presumably the code below will be preceded by and/or followed by definitions of other SrgsItem, SrgsOneOf, etc. to make up the grammar of your rule.
SrgsRuleRef garbageRuleRef = SrgsRuleRef.Garbage;
Mark