+Note [Dictionary Improvement]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In reduceContext, we first reduce equalities and then class constraints.
+However, the letter may expose further opportunities for the former. Hence,
+we need to go around again if dictionary reduction produced any dictionary
+bindings. The following example demonstrated the point:
+
+ data EX _x _y (p :: * -> *)
+ data ANY
+
+ class Base p
+
+ class Base (Def p) => Prop p where
+ type Def p
+
+ instance Base ()
+ instance Prop () where
+ type Def () = ()
+
+ instance (Base (Def (p ANY))) => Base (EX _x _y p)
+ instance (Prop (p ANY)) => Prop (EX _x _y p) where
+ type Def (EX _x _y p) = EX _x _y p
+
+ data FOO x
+ instance Prop (FOO x) where
+ type Def (FOO x) = ()
+
+ data BAR
+ instance Prop BAR where
+ type Def BAR = EX () () FOO
+
+During checking the last instance declaration, we need to check the superclass
+cosntraint Base (Def BAR), which family normalisation reduced to
+Base (EX () () FOO). Chasing the instance for Base (EX _x _y p), gives us
+Base (Def (FOO ANY)), which again requires family normalisation of Def to
+Base () before we can finish.
+
+