which can be `NULL` signifying "unknown". A `prop_err` flag set is
passed by reference. It has `Efail` set when an error is found, and
`Eretry` when the type for some element is set via propagation. If
which can be `NULL` signifying "unknown". A `prop_err` flag set is
passed by reference. It has `Efail` set when an error is found, and
`Eretry` when the type for some element is set via propagation. If