public override void AddCellBeforeModification(TerrainCell cellBefore)
    {
        TerrainCellAlterationPair pair;

        if (!_alterationPairs.TryGetValue(cellBefore.Position, out pair))
        {
            pair = new TerrainCellAlterationPair {
                Before = cellBefore.GetAlteration(true, false)
            };

            _alterationPairs.Add(cellBefore.Position, pair);
        }
    }