<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_article(doi, output_format='txt'):
 """Get the full body of an article from Elsevier. Parameters doi : str The doi for the desired article. output_format : 'txt' or 'xml' The desired format for the output. Selecting 'txt' (default) strips all xml tags and joins the pieces of text in the main text, while 'xml' simply takes the tag containing the body of the article and returns it as is . In the latter case, downstream code needs to be able to interpret Elsever's XML format. Returns ------- content : str Either text content or xml, as described above, for the given doi. """

    xml_string = download_article(doi)
    if output_format == 'txt' and xml_string is not None:
        text = extract_text(xml_string)
        return text
    return xml_string