'Sets the given usage key as the new root of the block structure.
Note: This method does *not* prune the rest of the structure. For
performance reasons, it is left to the caller to decide when exactly
to prune.
Arguments:
usage_key - The usage key of the block that is to be set as the
new root of the block structure.'

def set_root_block(self, usage_key):

    self.root_block_usage_key = usage_key
    self._block_relations[usage_key].parents = []
