# File warshall.rb, line 262
        def copy()
                temp = Relation.new([[]])
                @basis.each{|i| temp.basis.add(i)}
                @vor.each_key{|i| temp.vor[i]=@vor[i].copy}
                @nach.each_key{|i| temp.nach[i]=@nach[i].copy}
                return temp
        end