# File warshall.rb, line 215
        def addPaar!(x,y) 
          @basis.add(x) 
          @basis.add(y) 
          @nach[x] = Menge.new([y]) + @nach[x] 
          @nach[y] = Menge.new([]) if @nach[y]==nil 
          @vor[y] = Menge.new([x]) + @vor[y] 
          @vor[x] = Menge.new([]) if @vor[x]==nil 
                                        return self
     end