code: def get_abstract(doi):
      
      xml_string = download_article(doi)
      if xml_string is None:
          return None
      assert isinstance(xml_string, str)
      xml_tree = ET.XML(xml_string.encode('utf-8'), parser=UTB())
      if xml_tree is None:
          return None
      coredata = xml_tree.find('article:coredata', elsevier_ns)
      abstract = coredata.find('dc:description', elsevier_ns)
      abs_text = abstract.text
      return abs_text
docs: Get the abstract text of an article from Elsevier given a doi.