What to do with links to external proceedings, for conference series that are partially listed?

On the [the SIGGEN page](https://aclanthology.org/sigs/siggen/), *Third International Conference on Natural Language Generation* links to [this book](http://www.springer.com/computer/ai/book/978-3-540-22340-5). This is useful because then the ACL Anthology provides an index of the conference's history. In my documentation of the history of INLG, I've found several of these cases that are not yet listed in the Anthology. What to do with those?

Ideally I'd say the proceedings should not be linked directly, but rather there should be a proceedings page with relevant metadata, including separate links to all papers (if available). This way the external proceedings would be searchable within the ACL Anthology if you look for the metadata (I think?).

But if such a radical solution is not desired, it would just be nice to add the proceedings links. This would involve creating additional IDs in the Anthology. I could open a separate issue for this.

_Originally posted by @evanmiltenburg in https://github.com/acl-org/acl-anthology/issues/1539#issuecomment-906149787_