Multiple Inheritance

  The CREATE CLASS command can be used to create multiply inherited classes,
  as follows:

    CREATE CLASS <className> ;
       FROM <parentClass1>, <parentClass2> [, ...]

  To implement multiple inheritance correctly, Class(y) examines the
  ancestor classes of a multiply inherited class, and determines a single
  path through the ancestor tree. This affects which messages take
  precedence in the final class. It allows repeated inheritance, in
  which the same class occurs repeatedly in the ancestor tree. Class(y)
  treats repeatedly inherited classes as a single class. A diagram should
  help to explain this:

            Object
             |
             Box
             |
           Window
          /     \
    TextWindow   BorderedWindow
           \    /
         BordTextWindow

  The class BordTextWindow is multiply inherited from TextWindow and
  BorderedWindow. Assume that the BordTextWindow class was created with the
  following statement:

    CREATE CLASS BordTextWindow FROM TextWindow, BorderedWindow

  Class(y) will search for messages in the classes in the following
  sequence:

    BordTextWindow, TextWindow, BorderedWindow, Window, Box, Object

  Although the Window class is inherited from both branches of the tree, it
  is treated as a single class, ie. you do not end up with two sets of
  Window instance variables in the resulting class.

  The algorithm for determining a single path through the inheritance tree
  uses two rules (see Booch pg. 113):

  .  A class always has precedence over its superclass; ie. a superclass is
     never searched before any of its subclasses present in the ancestor
     tree of a multiply inherited class.

  .  Each class sets the precedence order of its direct superclasses.