def get_cited_dois(file):
    """
    Get the DOIs of the papers cited in a plaintext file. The file should \
            have one citation per line.

    .. note::

        This function is also used as a backend tool by most of the others \
        citations processors, to factorize the code.

    :param file: Either the path to the plaintext file or the content of a \
            plaintext file. It can also be a parsed list of plaintext \
            citations and, in this case, no preprocessing is done.
    :returns: A dict of cleaned plaintext citations and their associated DOI.
    """
    # If file is not a pre-processed list of plaintext citations
    if not isinstance(file, list):
        # It is either a path to a plaintext file or the content of a plaintext
        # file, we need some pre-processing to get a list of citations.
        plaintext_citations = get_plaintext_citations(file)
    else:
        # Else, we passed a list of plaintext citations.
        plaintext_citations = file
    dois = {}
    crossref_queue = []

    # Try to get the DOI directly from the citation
    for citation in plaintext_citations[:]:
        # Some citations already contain a DOI so try to match it directly
        matched_dois = doi.extract_from_text(citation)
        if len(matched_dois) > 0:
            # Add the DOI and go on
            dois[citation] = next(iter(matched_dois))
            continue
        # Same thing for arXiv id
        matched_arxiv = arxiv.extract_from_text(citation)
        if len(matched_arxiv) > 0:
            # Add the associated DOI and go on
            dois[citation] = arxiv.to_doi(next(iter(matched_arxiv)))
            continue
        # If no match found, stack it for next step
        # Note to remove URLs in the citation as the plaintext citations can
        # contain URLs and they are bad for the CrossRef API.
        crossref_queue.append(tools.remove_urls(citation))

    # Do batch with remaining papers, to prevent from the timeout of CrossRef
    for batch in tools.batch(crossref_queue, CROSSREF_MAX_BATCH_SIZE):
        batch = [i for i in batch]
        try:
            # Fetch results from CrossRef
            request = requests.post(CROSSREF_LINKS_API_URL, json=batch)
            for result in request.json()["results"]:
                # Try to get a DOI
                try:
                    dois[result["text"]] = result["doi"]
                except KeyError:
                    # Or set it to None
                    dois[result["text"]] = None
        except (RequestException, ValueError, KeyError):
            # If an exception occurred, set all the DOIs to None for the
            # current batch
            for i in batch:
                dois[i] = None
    return dois
Get the DOIs of the papers cited in a plaintext file. The file should \
            have one citation per line.

    .. note::

        This function is also used as a backend tool by most of the others \
        citations processors, to factorize the code.

    :param file: Either the path to the plaintext file or the content of a \
            plaintext file. It can also be a parsed list of plaintext \
            citations and, in this case, no preprocessing is done.
    :returns: A dict of cleaned plaintext citations and their associated DOI.