code: @SuppressWarnings("unchecked")
      private static <S extends Storable> ExistsFilter<S> getCanonical(ChainedProperty<S> property,
                                                                       Filter<?> subFilter,
                                                                       boolean not)
      {
          return (ExistsFilter<S>) cCanonical.put(new ExistsFilter<S>(property, subFilter, not));
      }
docs: Returns a canonical instance, creating a new one if there isn't one
  already in the cache.