sane_ty Dyadic (TyF t1 (TyF t2 td))
   | t1 == td && t2 == td  = True
sane_ty Dyadic (TyF t1 (TyF t2 td))
   | t1 == td && t2 == td  = True
sane_ty Dyadic (TyF t1 (TyF t2 td))
   | t1 == td && t2 == td  = True