[Haskell-beginners] MonadPlus

[Johann Bach]

I'm looking at Douglas Auclair's MonadPlus article, here
http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf

So consider an example like

t2 = do
  x <- [1,2,3]
  y <- [1,2]
  guard $ x+y > 2
  return (x,y)

This uses the list instance of MonadPlus.

Now, Douglas is talking about his own code, not the above example, but
his own code is similar. And he says "the entire computation is
chained by mplus". I'm confused because I thought it was chained by
>>=. In fact, I can't see where the above code makes use of mplus. The
definition of "guard" uses mzero.