+
+Note [Spontaneous solving and kind compatibility]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Note that our canonical constraints insist that only *given* equalities (tv ~ xi)
+or (F xis ~ rhs) require the LHS and the RHS to have exactly the same kinds.
+
+ - We have to require this because:
+ Given equalities can be freely used to rewrite inside
+ other types or constraints.
+ - We do not have to do the same for wanteds because:
+ First, wanted equations (tv ~ xi) where tv is a touchable unification variable
+ may have kinds that do not agree (the kind of xi must be a sub kind of the kind of tv).
+ Second, any potential kind mismatch will result in the constraint not being soluble,
+ which will be reported anyway. This is the reason that @trySpontaneousOneWay@ and
+ @trySpontaneousTwoWay@ will perform a kind compatibility check, and only then will
+ they proceed to @solveWithIdentity@.
+
+Caveat:
+ - Givens from higher-rank, such as:
+ type family T b :: * -> * -> *
+ type instance T Bool = (->)
+
+ f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
+ flop = f (...) True
+ Whereas we would be able to apply the type instance, we would not be able to
+ use the given (T Bool ~ (->)) in the body of 'flop'
+