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

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