with-fields := λ(: tt Type). (: (
   (match (slot( tt 'Fields_s )) (
      ()
      ( (TGround( 'Fields_s _ )) () )
      ( _ (
         (let class (with-only-class tt))
         (let tag (with-only-tag tt))
         (if (&&( (non-zero tag) (non-zero class) )) (
            (let ft (fields-of-tag( tag class )))
            (set tt (and( tt ft )))
         ) ())
      ))
   ))
   tt
) Type);