Tutorial questions

[Patrick Waugh]

Oh the TutorialMerge (http://mercurial.selenic.com/wiki/TutorialMerge)
page it says this:

---
Note that if you change your mind and you want to undo this merge
before comitting anything, just doing a hg revert -r2 --all  will only
revert the state of the files in the working dir to revision 2 and not
change the parents of the working dir back to a single parent (see
Revert). You should do a "hg update -C -r." instead to undo the merge
in this case.
---

And after reviewing the man page for update (hg -v help update), I do
not understand the "-r" as there is no revision number given.  Also,
if the first command was executed, given the dual parents, what would
happen then?  How could that be fixed?  It doesn't seem intuitive that
the revert command wouldn't work.  I'd like to understand the parents
thing better conceptually.

patrick