+Note [Avoid unecessary constraint simplification]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When inferring the type of a let-binding, with simplifyInfer,
+try to avoid unnecessariliy simplifying class constraints.
+Doing so aids sharing, but it also helps with delicate
+situations like
+ instance C t => C [t] where ..
+ f :: C [t] => ....
+ f x = let g y = ...(constraint C [t])...
+ in ...
+When inferring a type for 'g', we don't want to apply the
+instance decl, because then we can't satisfy (C t). So we
+just notice that g isn't quantified over 't' and partition
+the contraints before simplifying.
+
+This only half-works, but then let-generalisation only half-works.
+
+