clear region (unknown & occupied) inside bounding box in Octomap

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Hi all: I am using octomap in my project. I want to clear a region inside given bounding box in the 3D map constructed using Octomap. I know that there exists a service ("clear_bbx") in Octomap to clear a region inside bounding box, however this service only clears the occupied voxels but not the unknown region within bounding box.

I know that the unknown voxels are not initialized and therefore it (clearing of unknown) requires to iterate through them and thereafter further divide an unknown voxel (till it fits into bounding box or no need if it is already in bounding box) and set its occupancy value of FREE.

Need help on this Please. Thanks!