protected override bool TryGetPositions(T subKey, out IBitList positionMask)
        {
            if (subKeyPositions.TryGetValue(subKey, out positionMask))
            {
                positionMask = new BitList(positionMask);
                return(true);
            }

            positionMask = default(IBitList);
            return(false);
        }
 protected abstract bool TryGetPositions(T subKey, out IBitList positionMask);