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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 get_plaintext_citations(arxiv_id):
 """ Get the citations of a given preprint, in plain text. .. note:: Bulk download of sources from arXiv is not permitted by their API. \ You should have a look at http://arxiv.org/help/bulk_data_s3. :param arxiv_id: The arXiv id (e.g. ``1401.2910`` or ``1401.2910v1``) in \ a canonical form. :returns: A list of cleaned plaintext citations. """

    plaintext_citations = []
    # Get the list of bbl files for this preprint
    bbl_files = arxiv.get_bbl(arxiv_id)
    for bbl_file in bbl_files:
        # Fetch the cited DOIs for each of the bbl files
        plaintext_citations.extend(bbl.get_plaintext_citations(bbl_file))
    return plaintext_citations