#1, #3: Added docstrings to _path_to_id().
#1, #9: Added tests of the _path_to_id().
#1, #3: Added docstrings to _calc_dir_size().
#1, #9: Added unittests for _calc_dir_size()
#1: Removed unused import in __init__.py.
#1, #3: Added docstring to _remove_hairs(). Removed bugs.
#1, #9: Added unittests of the _remove_hairs().
#1, #3: Added docstrings to _add_order().
#1, #9: Added tests of the _add_order().