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	Bug in loop detection in TcSimplify
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The type-class context simplifier has been able to 
build recursive dictionaries for some time: co-induction.
That is, you can build a proof for constraint C by assuming
that C holds when proving the preconditions of C.

You need to be in -fallow-undecidable-instances land to 
make use of this: see comments with [RECURSIVE DICTIONARIES]
in TcSimplify.lhs.

Anyway, this is all fine, but I'd implemented it wrong!  You need
to be very careful with superclasses, or you can make a bogus
loop by mistake.  This commit fixes it; tests LoopOfTheDay{1,2,3}
will test it (thanks Ralf Laemmel).