mkTcSubCo co
   = ASSERT2( tcCoercionRole co == Nominal, ppr co)
     TcSubCo co
mkTcSubCo co
   = ASSERT2( tcCoercionRole co == Nominal, ppr co)
     TcSubCo co
mkTcSubCo co
   = ASSERT2( tcCoercionRole co == Nominal, ppr co)
     TcSubCo co