Return direct child of *element* having *child_tagname* or :class:`None`
    if no such child element is present.

def _child(element, child_tagname):
    """
    Return direct child of *element* having *child_tagname* or :class:`None`
    if no such child element is present.
    """
    xpath = './%s' % child_tagname
    matching_children = element.xpath(xpath, namespaces=nsmap)
    return matching_children[0] if len(matching_children) else None