Create a substring from a given UTF-8 string Note:
       positions are given in units of UTF-8 chars

def UTF8Strsub(utf, start, len):
    """Create a substring from a given UTF-8 string Note:
       positions are given in units of UTF-8 chars """
    ret = libxml2mod.xmlUTF8Strsub(utf, start, len)
    return ret