ftvs :: Subst.Zonk a => a -> TC (Set.Set TVar)
ftvs a = TC $
  do RW { .. } <- get
     Subst.ftvs rwSubst a
ftvs :: Subst.Zonk a => a -> TC (Set.Set TVar)
ftvs a = TC $
  do RW { .. } <- get
     Subst.ftvs rwSubst a
ftvs a = TC $
  do RW { .. } <- get
     Subst.ftvs rwSubst a