Simplifier bailing out?

[Mircea Draghicescu]

Not really a bug, but what does the "Simplifier still going after 4 iterations..." 
message mean?
For example, I get it when compiling the following with -O:

module Simplifier where
import Monad

i,j :: MonadPlus m => m a -> m a
i x = x
j x = i (x >>= return)

The message goes away after changing "MonadPlus" to "Monad"!
Thanks.

Mircea