That's not supported, and I don't know whether I want to - I've only added the combination rules for parity with other parser combinators, but am not a big fan of it.

What you can parse is a dsl::list(identifier + dsl::lit_c<'='> + dsl::p<value>, dsl::sep(dsl::comma)). The check that the value matches the given identifier, or that all identifiers parsed are valid is a semantic check that can be done in a later step, not a syntactic rule.

lexy is designed for parsing and that it can do a bit of semantic analysis is a coincidence and not what I consider to be best practices.