A query reads one kind of source (PLAN ยง6.8): its first source of a kind
fixes the kind, and each source of another kind is invalid, with a message
naming both kinds. Nodes of no kind, such as Unknown ones, are ignored.
Registered once, on the data product source.
A query reads one kind of source (PLAN ยง6.8): its first source of a kind fixes the kind, and each source of another kind is invalid, with a message naming both kinds. Nodes of no kind, such as Unknown ones, are ignored. Registered once, on the data product source.