code: public static <K, V> void putSetMult(Map<K, Set<V>> map, K key,
              Collection<? extends V> values) {
          for (V value : values) {
              putSet(map, key, value);
          }
      }
docs: Puts a collection of values into a map of key -> set of values.
  If the specified key is new, it creates an {@link ArrayList} as its
  value.
  @param map a {@link Map}.
  @param key a key.
  @param values a {@link Collection} of values.