def chartspan(cls, start, end):
        """
        Create a Lnk object for a chart span.

        Args:
            start: the initial chart vertex
            end: the final chart vertex
        """
        return cls(Lnk.CHARTSPAN, (int(start), int(end)))
Create a Lnk object for a chart span.

        Args:
            start: the initial chart vertex
            end: the final chart vertex