/** Return the derivation of at triple. The derivation is a List of DerivationRecords */
 @Override
 public Iterator<Derivation> getDerivation(Triple t) {
   if (derivations == null) {
     return new NullIterator<Derivation>();
   } else {
     return derivations.getAll(t);
   }
 }
 /** Log a dervivation record against the given triple. */
 @Override
 public void logDerivation(Triple t, Derivation derivation) {
   derivations.put(t, derivation);
 }