Get the text content of the unique child element under xtag with name eltname

def _get_unique_child_value(xtag, eltname):
    """Get the text content of the unique child element under xtag with name eltname"""
    xelt = _get_unique_child(xtag, eltname)
    if xelt is None:
        return None
    else:
        return xelt.text