The Constraint kind appears to lack an interface to an important capability that is already part of the type checker: constraint implication.  Namely, the ability to provide a witness for the statement "constraint c1 implies constraint c2" or, more importantly, "for all a, constraint (c1 a) implies constraint (c2 a)", where c1 and c2 are now constraint-valued type functions (and possibly even for constraint functions with multiple parameters).  It seems to me that this can follow the pattern of the "magic" Coercible type class and the (non-magic) Coercion data type; it provides the programmer with an analogous value to this example that can be obtained in apparently no other way.

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