guava/src/com/google/common/collect/Multisets.java
private static <E> boolean addAllImpl(Multiset<E> self, Multiset<? extends E> elements) {
    if (elements.isEmpty()) {
      return false;
    }
    elements.forEachEntry(self::add);
    return true;
  }
private static <E> boolean addAllImpl(Multiset<E> self, Multiset<? extends E> elements) {
    if (elements.isEmpty()) {
      return false;
    }
    elements.forEachEntry(self::add);
    return true;
  }
A specialization of {@code addAllImpl} for when {@code elements} is itself a Multiset.