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.