code: private Set<Tuple> getOnceTupleDefs( final List<VarDef> combinedVars)
      {
      try
        {
        return
          new HashSet<Tuple>(
            IteratorUtils.toList(
              IteratorUtils.transformedIterator(
                getOnceTuples(),
                tupleRef -> toTuple( combinedVars, tupleRef))));
        }
      catch( Exception e)
        {
        throw new IllegalStateException( "Invalid once-only tuple definition", e);
        }
      }
docs: Returns the set of once-only tuple definitions for this combiner.