[Info-ingres] E_US16D5 Table specified no longer exists

[Roy Hann]

This re Ingres SPARC SOLARIS Version II 11.0.0 (su9.us5/100)

* alter table medical drop column m_length cascade \g
Executing . . .

E_US16D5 Table specified no longer exists.
    (Thu Sep 26 10:34:53 2019)

A bit of googling tells me this error has cropped up once or twice in
the past but none of the past cases seem relevant, e.g. there are no
constraints involved.

Obviously I am wondering how to get past it, but my question here is why
I would ever get an error message that something "no longer" exists?
Surely to the DBMS a thing exists or it doesn't. Why the reference
to a past state of which it should be (or at least should seem to be)
oblivious?

--Roy