Theory of operation of collation patch

Is there any documentation of $SUBJECT?   Because the more I look at
this patch the more I think it's misdesigned; either that or I
fundamentally misunderstand what it's trying to do.  I complained
yesterday about why the planner wasn't using indcollation to identify
the sort order of an index.  I now think that the reason it doesn't
obviously fail to fail is that indcollation is dead code, and so is
approximately 99% of what you added to the planner, because two
expressions that are equal() must necessarily have the same collation
property.  Tracking the collation as a separate property of a pathkey
is thus a useless activity.  If this conclusion isn't correct, please
explain why not.
        regards, tom lane