def qn(tag):
    """
    Stands for 'qualified name', a utility function to turn a namespace
    prefixed tag name into a Clark-notation qualified tag name for lxml. For
    example, ``qn('p:cSld')`` returns ``'{http://schemas.../main}cSld'``.
    Source: https://github.com/python-openxml/python-docx/
    """
    nsmap = {'w':
        'http://schemas.openxmlformats.org/wordprocessingml/2006/main'}
    prefix, tagroot = tag.split(':')
    uri = nsmap[prefix]
    return '{{{}}}{}'.format(uri, tagroot)
