def _get_subsection_of_block(usage_key, block_structure):
    """
    Finds subsection of a block by recursively iterating over its parents
    :param usage_key: key of the block
    :param block_structure: block structure
    :return: sequential block
    """
    parents = block_structure.get_parents(usage_key)
    if parents:
        for parent_block in parents:
            if parent_block.block_type == 'sequential':
                return parent_block
            else:
                return _get_subsection_of_block(parent_block, block_structure)