This is likely a complex issue resulting from multiple bugs.

• POSL parser cannot deal with certain slot uses in KBs such as
  KB: q(a->b) Query q(!?SQuery).
  This does work with KB in RuleML syntax and the query in POSL.
• RuleML parser also has some troubles with rest slot variables. For example, the KB: r(![a->b]), Query: r(![a->?X]) fails in all possible combinations of RuleML or POSL syntax.

(perhaps only plex-valued rest variables, not plexes themselves, are intended by OO jDREW after "|" and "!")

See http://ruleml.org/1.0/exa/Issues/resl-test.ruleml