-A SpecPrag has a field for "constant dicts" in the RULE, but I think
-it's pretty useless. See the place where it's generated in TcBinds.
-TcSimplify will discharge a constraint by binding it to, say,
-GHC.Base.$f2 :: Eq Int, withour putting anything in the LIE, so this
-dict won't show up in the const-dicts field. It probably doesn't matter,
-because the rule will end up being something like
- f Int GHC.Base.$f2 = ...
-rather than
- forall d. f Int d = ...
-The latter is more general, but in practice I think it won't make any
-difference.
+When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict,
+which is presumably in scope at the function definition site, we can quantify
+over it too. *Any* dict with that type will do.
+
+So for example when you have
+ f :: Eq a => a -> a
+ f = <rhs>
+ {-# SPECIALISE f :: Int -> Int #-}
+
+Then we get the SpecPrag
+ SpecPrag (f Int dInt) Int
+
+And from that we want the rule
+
+ RULE forall dInt. f Int dInt = f_spec
+ f_spec = let f = <rhs> in f Int dInt
+