How to use the vivisect.const function in vivisect

def get_predecessor_basic_blocks(self, bb):
        if bb.va in self._pred_cache:
            for nbb in self._pred_cache[bb.va]:
                yield nbb
            return

        predecessors = []
        for xref in get_all_xrefs_to(self.vw, bb.va):
            try:
                pred = self.bb_by_end[xref[vivisect.const.XR_FROM]]
                yield pred
                predecessors.append(pred)
            except KeyError:
                # if we have a jump to the import table,
                # the target of the jump is not a basic block in the function.
                continue

        self._pred_cache[bb.va] = predecessors

def get_successor_basic_blocks(self, bb):
        if bb.va in self._succ_cache:
            for nbb in self._succ_cache[bb.va]:
                yield nbb
            return

        successors = []
        next_va = bb.va + bb.size
        op = get_prev_opcode(self.vw, next_va)
        for xref in get_all_xrefs_from(self.vw, op.va):
            try:
                succ = self.bb_by_start[xref[vivisect.const.XR_TO]]
                yield succ
                successors.append(succ)
            except KeyError:
                # if we have a jump to the import table,
                # the target of the jump is not a basic block in the function.
                continue

        self._succ_cache[bb.va] = successors