; remove the father of ares
>(setf (get 'father 'ground) '(3))
(3)

>(goal '(grandparent x (harmonia)))
  1> (GOAL (GRANDPARENT X (HARMONIA)))
    2> (GOAL (PARENT Z (HARMONIA)))
      3> (GOAL (FATHER Z (HARMONIA)))
      <3 (GOAL ((Z ARES)))
    <2 (GOAL ((Z ARES)))
    2> (GOAL (PARENT X (ARES)))
      3> (GOAL (FATHER X (ARES)))
      <3 (GOAL NIL)
      3> (GOAL (MOTHER X (ARES)))
      <3 (GOAL ((X HERA)))
    <2 (GOAL ((X HERA)))
  <1 (GOAL ((X HERA)))
((X HERA))