Hi, I'm working on finalizing an extension to spirit/qi in which I need to extract an attribute from a fusion sequence in a parser context. The problem I'm facing is that this parser attribute can be an unused type or a valid fusion sequence. Just to illustrate my problem: template <typename Elements,typename Attribute> struct parser_dispachter { template <typename Index> void parse(Index &idx, Attribute &attr) { fusion::at_c<Index::value>(Elements).parse(fusion::at_c<Index::value>(attr)); } }; I actually have two cases: 1. Attribute = unused_type -> at_c replacement should return unused 2. Attribute = fusion sequence -> at_c replacement should return the corresponding attribute reference. What would be the best solution to extract the give attribute ? Regards, Thomas Bernard