The diverse inheritance mechanisms provided by Smalltalk, Beta, and CLOS are interpreted as different uses of a single underlying construct. Smalltalk a.nd Beta differ primarily in the direction of class hierarchy growth. These inheritance mechanisms are subsumed in a new inheritance model ba.sed on composition of mixins, or abstract subclasses. This form of inheritance can also encode a CLOS multiple-inherita.nce hierarchy, although changes to the encoded hierarchy that would violate

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... ion are difficult. Practical applica.tion of mixin-based inheritance is illustrated in a sketch of an extension to Modula-3.