external_bodies :: [Rule] -> [Atom] -> [[Literal]]
-- returns the external bodies
external_bodies p u =
  [ body r |  r <- p, Prelude.elem (kopf r) u, Prelude.null (intersect (pbody r) u) ]
external_bodies :: [Rule] -> [Atom] -> [[Literal]]
external_bodies p u =
  [ body r |  r <- p, Prelude.elem (kopf r) u, Prelude.null (intersect (pbody r) u) ]
external_bodies p u =
  [ body r |  r <- p, Prelude.elem (kopf r) u, Prelude.null (intersect (pbody r) u) ]