I don't get how subclasses are supersets of their parents. The set of instances of a subclass is a (usually proper) subset of the set of instances of its superc

I don't get how subclasses are supersets of their parents. The set of instances of a subclass is a (usually proper) subset of the set of instances of its superclass (in any mainstream class-OO language). Also, writing a subclass definition can be seen as adding constraints (of the form "must also have a bla() method returning an int").