'Arguments:
root_block_usage_key (UsageKey) - The usage_key for the root
of the block structure that is being accessed.
modulestore (ModuleStoreRead) - The modulestore that
contains the data for the xBlock objects corresponding to
the block structure.
cache (django.core.cache.backends.base.BaseCache) - The
cache to use for storing/retrieving the block structure\'s
collected data.'

def __init__(self, root_block_usage_key, modulestore, cache):

    self.root_block_usage_key = root_block_usage_key
    self.modulestore = modulestore
    self.store = BlockStructureStore(cache)
