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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def unpack(src_dir, dst_dir):
    '''Move everything under `src_dir` to `dst_dir`, and delete the former.'''
    for dirpath, dirnames, filenames in os.walk(src_dir):
        subdir = os.path.relpath(dirpath, src_dir)
        for f in filenames:
            src = os.path.join(dirpath, f)
            dst = os.path.join(dst_dir, subdir, f)
            os.renames(src, dst)
        for n, d in reversed(list(enumerate(dirnames))):
            src = os.path.join(dirpath, d)
            dst = os.path.join(dst_dir, subdir, d)
            if not os.path.exists(dst):
                # Directory does not exist in destination,
                # rename it and prune it from os.walk list.
                os.renames(src, dst)
                del dirnames[n]
    # Cleanup.
    for dirpath, dirnames, filenames in os.walk(src_dir, topdown=True):
        assert not filenames
        os.rmdir(dirpath)