Vitalie is in a better position to comment, but I think I can explain
poly-mode.

Per design, each buffer in Emacs has exactly one major mode associated with
it.  This is a problem for Sweave documents which really have parts of the
buffer in one major mode (R) and parts in another (Latex).  This is
currently supported by "noweb" mode which is responsible for switching
major modes around when the point moves around in the buffer.  poly-mode is
a newer (and hopefully better, I have not yet tested it) mode that supports
this and - based on the name - is probably much more general than
noweb-mode.

This issue (multiple major modes in one buffer) is behind the issues of
region selection have problems covering both R mode and Latex mode parts of
the buffer, as well as other issues related to editing Sweave documents.

My guess is that when poly-mode has matured and been thouroughly tested,
that no web-mode will disappear from ESS.

"We" should put together a short document on recent improvements/changes in
ESS.  I agree it can be hard to follow as a casual user, who do not want to
read the manual again and again. I might be willing to do so, but I need to
actually experiment with the changes, so perhaps someone else will beat me
to it.

Kasper

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