how does a CAF become unreachable?

[Norman Ramsey]

I spent the afternoon spelunking through some code and the Commentary,
and I'm wondering how a CAF becomes unreachable.  I gather it might
have to do with GHC floating a static expression out of a context
until it becomes a CAF, but I'm still not seeing how a CAF could
at one point be reachable, then dynamically become unreachable.

Can anyone show an example?


Norman