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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 afs_url(file_path):
 """Convert a file path to a URL pointing to its path on AFS. If ``file_path`` doesn't start with ``/opt/cds-invenio/``, and hence is not on AFS, it returns it unchanged. The base AFS path is taken from the Flask app config if present, otherwise it falls back to ``/afs/cern.ch/project/inspire/PROD``. """

    default_afs_path = '/afs/cern.ch/project/inspire/PROD'
    afs_path = current_app.config.get('LEGACY_AFS_PATH', default_afs_path)

    if file_path is None:
        return

    if file_path.startswith('/opt/cds-invenio/'):
        file_path = os.path.relpath(file_path, '/opt/cds-invenio/')
        file_path = os.path.join(afs_path, file_path)
        return urllib.parse.urljoin('file://', urllib.request.pathname2url(file_path.encode('utf-8')))

    return file_path