code: public static <K> Matcher<Multimap<K, ?>> keyWithSize(final K element, final int size) {
          return new IsMultimapKeyWithCollectionSize(element, size);
      }
docs: Creates a matcher for {@linkplain Multimap} matching when the examined object <code>K</code> in the key set has
  <code>size</code> elements in the retained {@linkplain Collection}.