code: public Collection<Set<Integer>> valueSets() {
          // An IntSet _is_ a Set<Integer>, but the JVM doesn't recognize
          // covaraince in the return type generics, so we must cast
          Collection<?> c = map.valueCollection();
          @SuppressWarnings("unchecked")
          Collection<Set<Integer>> c2 = (Collection<Set<Integer>>)c;            
          return c2;
      }
docs: {@inheritDoc}