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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 save_trajs(trajs, fn, meta, key_to_path=None):
 """Save trajectory-like data Data is stored in individual numpy binary files in the directory given by ``fn``. This method will automatically back up existing files named ``fn``. Parameters trajs : dict of (key, np.ndarray) Dictionary of trajectory-like ndarray's keyed on ``meta.index`` values. fn : str Where to save the data. This will be a directory containing one file per trajectory meta : pd.DataFrame The DataFrame of metadata """

    if key_to_path is None:
        key_to_path = default_key_to_path

    validate_keys(meta.index, key_to_path)
    backup(fn)
    os.mkdir(fn)
    for k in meta.index:
        v = trajs[k]
        npy_fn = os.path.join(fn, key_to_path(k))
        os.makedirs(os.path.dirname(npy_fn), exist_ok=True)
        np.save(npy_fn, v)