Returns the markable object for the supplied identifier
        @type mark_id: string
        @param mark_id: term identifier

def get_mark(self, mark_id):
        """
        Returns the markable object for the supplied identifier
        @type mark_id: string
        @param mark_id: term identifier
        """
        if mark_id in self.idx:
            return Cmarkable(self.idx[mark_id], self.type)
        else:
            return None