[yawaramin]

In Java, you can have:

    class Set<A> { public Set<A> union(Collection<A> c) { ... } }

    class SortedSet<A> extends Set<A> {
      @Override public SortedSet<A> union(Collection<A> c) { ... }
    }

In other words, the more specific type overrides the operations of the more general type into more specialised operations.