Uni-directional coercions, or representational subtyping relations.

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This package provides Attenuations, which act as uni-directional coercions, or representational subtyping relations.

Like Coercions, these can be used directly to coerce between their type parameters at zero runtime cost, or lifted to operate over "larger" types.

Unlike Coercions, these can be used only in one direction, from a "weaker" type to a "stronger" type. Accordingly, they have additional restrictions on lifting through type constructors: they must respect the variance (covariance or contravariance) of the type being lifted. Lifting an Attenuation covariantly lets you coerce from, e.g., [Fin n] -> [Int], but not vice versa; while lifting contravariantly lets you coerce from e.g. Op Bool Int -> Op Bool (Fin n), but not vice versa.