[1] Abadi, M. and Cardelli, L.: An Imperative Object Calculus, Theory and Practice of Object Systems, Vol.1, No.3, 1995, pp.151-166.
[2] Cook, R.W., Hill L.W., Canning, S.P.: Inheritance is Not Subtyping, Conference Record of the 17th Annual ACM Symposium on Principles of Programming Languages, 1990, pp.125-135.