# File Warshall.rb, line 272
        def vereinigt(rel)
    erg = Relation.new([[]])
    erg.basis!(@basis + rel.basis)
          erg.each{|i| erg.nach[i] = (@nach[i]==nil ? Menge.new([]) : @nach[i]) + rel.nach[i] }
    erg.each{|i| erg.vor[i]  = (@vor[i]==nil ? Menge.new([]) : @vor[i])  + rel.vor[i] }
    return erg
  end