How to assert that a country has one president

Hello,

We're trying to define an axiom to ensure a singular cardinality as follows:

;; BEGIN PowerLoom
(defconcept Person)
(defconcept Country)
(defrelation presidentOf ((?p Person) (?c Country)))

(assert (forall (?c ?p1 ?p2)
                 (=> (and (presidentOf ?p1 ?c)
                              (presidentOf ?p2 ?c))
                       (= ?p1 ?p2))))
;; END PowerLoom



The following error results when the above are entered into the PowerLoom command prompt:

 Internal Error: OOPS -- BUG IN 'copy-description'edu.isi.stella.StellaException: OOPS -- BUG IN 'copy-description'
      at edu.isi.powerloom.logic.Logic.copyPropositionArgument(Logic.java:29501)
      at edu.isi.powerloom.logic.Proposition.copyProposition(Proposition.java:635)
      at edu.isi.powerloom.logic.Logic.copyPropositionArgument(Logic.java:29480)
      at edu.isi.powerloom.logic.Proposition.copyProposition(Proposition.java:635)
      at edu.isi.powerloom.logic.Logic.copyPropositionArgument(Logic.java:29480)
      at edu.isi.powerloom.logic.Proposition.copyProposition(Proposition.java:635)
      at edu.isi.powerloom.logic.Proposition.createForwardChainingIndex(Proposition.java:4578)
      at edu.isi.powerloom.logic.Proposition.deriveComplexForwardRule(Proposition.java:4571)
      at edu.isi.powerloom.logic.Proposition.deriveOneSatelliteRule(Proposition.java:4954)
      at edu.isi.powerloom.logic.Proposition.deriveSatelliteRulesForGoalP(Proposition.java:4886)
      at edu.isi.powerloom.logic.Proposition.deriveSatelliteRules(Proposition.java:4796)
      at edu.isi.powerloom.logic.Proposition.runGoesTrueDemons(Proposition.java:11617)
      at edu.isi.powerloom.logic.Proposition.updateLinksAndTimestamps(Proposition.java:11492)
      at edu.isi.powerloom.logic.Proposition.assignTruthValue(Proposition.java:11469)
      at edu.isi.powerloom.logic.Proposition.updatePropositionTruthValue(Proposition.java:11277)
      at edu.isi.powerloom.logic.Proposition.helpUpdateTopLevelProposition(Proposition.java:9493)
      at edu.isi.powerloom.logic.Logic.updateTopLevelProposition(Logic.java:9231)
      at edu.isi.powerloom.logic.Logic.updateProposition(Logic.java:9260)
      at edu.isi.powerloom.logic.Logic.smartUpdateProposition(Logic.java:9406)
      at edu.isi.powerloom.logic.Logic.renamed_Assert(Logic.java:9427)
      at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
      at java.lang.reflect.Method.invoke(Unknown Source)
      at edu.isi.stella.javalib.Native.funcall(Native.java:608)
      at edu.isi.stella.Stella.apply(Stella.java:8401)
      at edu.isi.stella.Cons.evaluateConsTree(Cons.java:8355)
      at edu.isi.stella.Stella_Object.evaluate(Stella_Object.java:4351)
      at edu.isi.powerloom.logic.Logic.evaluateLogicCommand(Logic.java:31552)
      at edu.isi.powerloom.logic.Logic.logicCommandLoop(Logic.java:31448)
      at edu.isi.powerloom.logic.Logic.powerloom(Logic.java:37358)
      at edu.isi.powerloom.PowerLoom.main(PowerLoom.java:108)

Any advice is greatly appreciated.
Cameron Ross.

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