Pruned branches cells are cells that represent deleted subtrees of tree of cells, see the corresponding page.May have any level between 1 and 3, which is equal to the level of a root of the pruned subtree plus one.
Each of a pruned branch cell serialized as follows.
The 1-byte tag that always equals 1.
The byte that contains the pruned branch level mask, 1 <= mask <= 7. The number of hashes stored in the pruned branch cell
is equal to the number of 1-bits in mask.
Next, h * 32 bytes store representations hashes of roots of the pruned subtrees starting
from the last one. For example, if the mask equals 3 (binary 011), then the pruned tree cell contains representation hashes of the second
and the first pruned subtrees in sequential order, but not the third one.
Finally, h * 2 bytes store the depths of the pruned subtrees starting from the last one.
