+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'.
+
+