result.cleanStaleKnowledge(vnaFrameAfter);
if (!vnaFrameAfter.isTop()) result.cleanStaleKnowledge(vnaFrameAfter);
	public void transfer(BasicBlock basicBlock, InstructionHandle end, IsNullValueFrame start, IsNullValueFrame result)	throws DataflowAnalysisException {		startTransfer();		super.transfer(basicBlock, end, start, result);		endTransfer(basicBlock, end, result);		ValueNumberFrame vnaFrameAfter = vnaDataflow.getFactAfterLocation(Location.getLastLocation(basicBlock));		// purge stale information		result.cleanStaleKnowledge(vnaFrameAfter);	}