+canSolve :: CtFlavor -> CtFlavor -> Bool
+-- canSolve ctid1 ctid2
+-- The constraint ctid1 can be used to solve ctid2
+canSolve (Given {}) _ = True
+canSolve (Derived {}) (Wanted {}) = True
+canSolve (Derived {}) (Derived {}) = True
+canSolve (Wanted {}) (Wanted {}) = True
+canSolve _ _ = False
+