Ticket 1031 workaround?

Hello folks, I seem to have hit this problem.. http://hackage.haskell.org/trac/ghc/ticket/1031 (I think, at least I'm getting a very similar incomprehensible error message :-) I tried using bang patterns instead of `seq` like this:

let a0_ = f a0 a in a0_ `seq` (# l,hl,a0_,r,hr #)

becomes ..

let !a0_ = f a0 a in (# l,hl,a0_,r,hr #)

but I still get the same error. Could someone who knows explain exactly what the problem is and what the workaround is (if there is one)? Thanks -- Adrian Hey