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	Fix a small quantification bug
	------------------------------

We were quantifying over too few type variables, because fdPredsOfInsts was
being too eager to discard predicates. This only affects rather obscure
programs.  Here's the one Iavor found:

	class C a b where f :: a -> b
	g x = fst (f x)

We want to get the type
   	g :: forall a b c.  C a (b,c) => a -> b
but GHC 6.0 bogusly gets
   	g :: forall a b.  C a (b,()) => a -> b

A test is in should_compile/tc168