"whitelisted symbol" rules are too restrictive, not allowing building up rules from smaller rules.

[umanwizard]

Attempted this solution to chapter 8, ex. 1:

[
 [(prefriends ?p1 ?p2)
  [?m :movie/director ?p1]
  [?m :movie/cast ?p2]
 ]
 [(prefriends ?p1 ?p2)
  [?m :movie/cast ?p1]
  [?m :movie/director ?p2]
 ]
 [(prefriends ?p1 ?p2)
  [?m :movie/cast ?p1]
  [?m :movie/cast ?p2]
 ]
 [(friends ?p1 ?p2)
  (prefriends ?p1 ?p2)
  [(not= ?p1 ?p2)]
 ]
]

which was not accepted since prefriends is not a whitelisted symbol.