A message to Haskell Cafe from Grzegorz Chrupala made me realise that
GHC was not handling implicit parameters correctly, when it comes to
choosing the variables to quantify, and ambiguity tests.  Here's the 
note I added to TcSimplify:

Note [Implicit parameters and ambiguity] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What type should we infer for this?
	f x = (show ?y, x::Int)
Since we must quantify over the ?y, the most plausible type is
	f :: (Show a, ?y::a) => Int -> (String, Int)
But notice that the type of the RHS is (String,Int), with no type 
varibables mentioned at all!  The type of f looks ambiguous.  But
it isn't, because at a call site we might have
	let ?y = 5::Int in f 7
and all is well.  In effect, implicit parameters are, well, parameters,
so we can take their type variables into account as part of the
"tau-tvs" stuff.  This is done in the function 'FunDeps.grow'.


The actual changes are in FunDeps.grow, and the tests are
	tc219, tc219