Type Inheritance & Relational Theory: Subtypes, Supertypes, and Substitutability by C. J. Date really nails down type inheritance, and shows how almost all OOPLs have it wrong. e.g., the "diamond problem" isn't an actual problem, just bad implementation.
The Shlaer-Mellor Commons needs to address this in the Shlaer-Mellor generalization rules. Finally, something juicy to work on!
#ShlaerMellor
#RelationalDataModel