(tyvars, rho) -> tcInstTyVars tyvars `thenNF_Tc` \ (tyvars', _, tenv) ->
tcSplitRhoTy (substTy tenv rho) `thenNF_Tc` \ (theta, tau) ->
returnNF_Tc (tyvars', theta, tau)
(tyvars, rho) -> tcInstTyVars tyvars `thenNF_Tc` \ (tyvars', _, tenv) ->
tcSplitRhoTy (substTy tenv rho) `thenNF_Tc` \ (theta, tau) ->
returnNF_Tc (tyvars', theta, tau)