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abstract class IntList {
    abstract returnType methodName(parameter_list); // returnType may be Void
}

class EmptyIntList extends IntList {
    static EmptyIntList ONLY = new EmptyIntList(); // Singleton
    private EmptyIntList() {}
 
    returnType methodName(parameter_list) {
        // base case code
    }
}

class ConsIntList extends IntList {
    int first;
    IntList rest;
    returnType methodName(parameter_list) {
        // ... first ...
        // ... rest.methodName(parameter_list)...
    }

}