code: def contrib_email(contrib_tag):
      
      email = []
      for email_tag in extract_nodes(contrib_tag, "email"):
          if email_tag.parent.name != "aff":
              email.append(email_tag.text)
      return email if len(email) > 0 else None
docs: Given a contrib tag, look for an email tag, and
      only return the value if it is not inside an aff tag