An unresolved !Name(arg) tagged value (CASC.md §7.5). The grammar only recognizes "a tag plus one parenthesized argument" -- arg is already a fully evaluated ordinary value (recursively -- it may itself be a nested ref); giving name meaning (built-in int/ float/bool/duration/bytes, or a consumer-registered tag) is Cooper.Resolver's job, via the :tags option (see Cooper).