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;