This looks like a promising definition but I see some things that may cause
Rather than define the collection as multiple sets (one of identifiers, one
of links, and one of metadata), why not just define it as a set of digital
objects (each of which has an identifier, some link pointing to it, and
some descriptive metadata). I might add some caveat like "at a particular
point in time" so that there is enough flexibility to admit that
collections tend to change over time.