Passing a $url parameter to every function was error-prone
and having {url} field for a short-lived object is appropriate.
This matches the version of IMAPTracker posted by
Eric W. Biederman on 2020-05-15 at:
https://public-inbox.org/meta/87ftc0c3r4.fsf_-_@x220.int.ebiederm.org/
The version I originally imported was based on the one
posted on 2019-10-09:
https://public-inbox.org/meta/874l0i9vhc.fsf_-_@x220.int.ebiederm.org/