public interface HigherOrderCollection {
    /** Add an object to this collection */
    void add(Object o);

    /** Apply f to each element of this collection. */
    void doEach(Function f);

    /** Apply f to each element in this collection, and add the result
        to target. */
    void map(HigherOrderCollection target, Function f);

    /** Add all elements satisfying pred into the target collection. */
    void filter(HigherOrderCollection target, Function pred);

    // ... other operations
}