<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_abstract(pubmed_id, prepend_title=True):
 """Get the abstract of an article in the Pubmed database."""

    article = get_article_xml(pubmed_id)
    if article is None:
        return None
    return _abstract_from_article_element(article, prepend_title)