isSystemFunction

True when this call names a system function, which no catalog defines.

The @ namespace is reserved -- a2ui_protocol.md's System Namespace Rule gives it to system context evaluations and bars catalogs from defining into it, which is why checkEntityNames refuses such a definition. But v1.0 populates it with exactly one name: FunctionCall is a oneOf over a catalog function and IndexSystemFunction, whose call is the constant @index.

So a reserved prefix is not a system function. Any other @-prefixed name is a call on nothing — no catalog may define it and no system function answers to it — and it must fail catalog resolution rather than skip it, because that is where the message saying so comes from.