The theory of classification part 16: rules of extension and the typing of inheritance

The mechanism of inheritance, keeping in view the constraints on what may or may not be added to a class during inheritance, is examined. It is considered reasonable to to think of the merger of two record types as the set union of their respective sets of maplets. In the subtyping model, it is suggested to consider the possibility that fields of different types may be merged. It is possible to construct Derived record types using simple set union ∩ to merge the two sets of type signatures in the corresponding Base and Extra type-records.