How to use the metar.Datatypes.distance.legal_gtlt function in metar

self._units = "M"
        else:
            if units.upper() not in distance.legal_units:
                raise UnitsError("unrecognized distance unit: '" + units + "'")
            self._units = units.upper()

        try:
            if value.startswith("M"):
                value = value[1:]
                gtlt = "<"
            elif value.startswith("P"):
                value = value[1:]
                gtlt = ">"
        except:
            pass
        if gtlt and gtlt not in distance.legal_gtlt:
            raise ValueError(
                "unrecognized greater-than/less-than symbol: '" + gtlt + "'"
            )
        self._gtlt = gtlt
        try:
            self._value = float(value)
            self._num = None
            self._den = None
        except ValueError:
            mf = FRACTION_RE.match(value)
            if not mf:
                raise ValueError("distance is not parseable: '" + str(value) + "'")
            df = mf.groupdict()
            self._num = int(df["num"])
            self._den = int(df["den"])
            self._value = float(self._num) / float(self._den)