takes one prefix line from the turtle file and binds the namespace
        to the class

        Args:
            line: the turtle prefix line string

def _add_ttl_ns(self, line):
        """ takes one prefix line from the turtle file and binds the namespace
        to the class

        Args:
            line: the turtle prefix line string
        """
        lg = logging.getLogger("%s.%s" % (self.ln, inspect.stack()[0][3]))
        lg.setLevel(self.log_level)

        lg.debug("line:\n%s", line)
        line = str(line).strip()
        # if the line is not a prefix line exit
        if line is None or line == 'none' or line == '' \
                or not line.lower().startswith('@prefix'):
            return
        # parse the turtle line
        line = line.replace("@prefix","",1).strip()
        if line.endswith("."):
            line = line[:-1]
        prefix = line[:line.find(":")].strip()
        uri = self.clean_iri(line[line.find(":")+1:].strip())
        # add the namespace to the class
        lg.debug("\nprefix: %s  uri: %s", prefix, uri)
        self.bind(prefix, uri, override=False, calc=False)