                  E/R-Diagramm Notation
                  =====================

   *---           Konkrete Einzel-Relation: +Bool, +Number, +Symbol, +Link
   @---           Konkrete Mehrfach-Relation: +List, +Bag
   O---           Virtuelle Mehrfach-Relation: +Ref


    Hook
     |
   --*--+
      \ |
        @--       Hook-Relation


   Beispiele:

      |
      *-- nr         +Number
      |

      |     |
      *-----+        +Link
      |     |

      |     |
      @-----+        +List +Link
      |     |

      |     |
      *-----O        +Ref +Link
      |     |

      |     |
      @-----O        +List +Ref +Link
      |     |

      |     |
      *-----*        +Joint   <->   +Joint
      |     |

      |     |
      @-----*        +List +Joint   <->   +Joint
      |     |
