example taken from: http://gauss.gwydiondylan.org/books/drm/drm_50.html

         Object
           ^
           |
        LifeForm 
         ^    ^
        /      \
   Sentient    BiPedal
      ^          ^
      |          |
 Intelligent  Humanoid
       ^        ^
        \      /
         Vulcan

 define class <sentient> (<life-form>) end class;
 define class <bipedal> (<life-form>) end class;
 define class <intelligent> (<sentient>) end class;
 define class <humanoid> (<bipedal>) end class;
 define class <vulcan> (<intelligent>, <humanoid>) end class;