Use a Representaional coercion for data families
When we have data instance T (a,b) = MkT a b we make a represntation type data TPair a b = MkT a b plus an axiom to connect the two ax a b :: T (a,b) ~R TPair a b Previously this was a Nominal equality, and that worked ok but seems illogical since Nominal equalities are between types that the programmer thinks of as being equal. But TPair is not visible to the programmer; indeed we call it the "representation TyCon". So a Representational equality seems more suitable here.
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