[$] Distinguishing Debian testing from unstable

Sometimes, the smallest changes create the longest discussions.  As a case
in point, a proposal to make a one-line change in an informational text
file on systems running the Debian unstable distribution has blown up into
an interminable and sometimes unfriendly debate.  At its core, though, this
discussion comes down to a seemingly simple question: should a program be
able to determine whether it is running on a Debian testing or unstable
system?

https://lwn.net/Articles/984635/