'Return a conversion command that copies files and appends
`tag` to the copy.'

def tagged_copy_cmd(self, tag):

    if re.search('[^a-zA-Z0-9]', tag):
        raise ValueError(u"tag    '{0}'    must    only    contain    letters    and    digits".format(tag))
    stub = os.path.join(_common.RSRC, 'convert_stub.py').decode('utf-8')
    return u'{}    {}    $source    $dest    {}'.format(shell_quote(sys.executable), shell_quote(stub), tag)
