public BackwardDataflowAnalysis(ReverseDepthFirstSearch rdfs) { if (rdfs == null) throw new IllegalArgumentException();
public BackwardDataflowAnalysis(ReverseDepthFirstSearch rdfs, DepthFirstSearch dfs) { if (rdfs == null || dfs == null) throw new IllegalArgumentException();
	public BackwardDataflowAnalysis(ReverseDepthFirstSearch rdfs) {		if (rdfs == null) throw new IllegalArgumentException();		this.rdfs = rdfs;	}