We describe Permco, a permissive approach which allows covariant overriding of instance variables and any overriding of instance methods. Subclasses are considered subtypes. Instance method access is treated by considering the types of all methods defined for the class of the receiver and all its possible subclasses. Instance variable access is treated by considering the types of the instance variable in all possible subclasses. Thus, Permco is permissive at the point of redefinition and restrictive at the point of call or access. Closed types describe values which may belong to a class but not to its subclasses, and allow a more precise description of types. Result types may depend on receiver or argument types. This paper introduces Permco in terms of an example and compares with related approaches. It then demonstrates its soundness through a subject reduction theorem
