How many UAs make it possible, let alone easy, for authors to
edit References or In-Reply-To fields?
For that matter, how many permit a user to select multiple
messages to compose a response citing all of the selected
messages (including appropriate setting of In-Reply-To and
References fields)?
This is probably an area where some advice to UA authors
is called for.
seems like we need some experiments before making such recommendations.
we can usually guess how a protocol will function without extensive
real-world testing. this doesn't work nearly as well for user
interfaces. humans are a lot less predictable than machines (alas :)