code: public static <K, V> void putSetAll(Map<K, Set<V>> map, Map<K, Set<V>> other) {
          for (Map.Entry<K, Set<V>> me : other.entrySet()) {
              putSetMult(map, me.getKey(), me.getValue());
          }
      }
docs: Copies all of the mappings from the second map to the first. The value
  for each key is the union of the first and second maps' values for that
  key.
  @param <K>
  @param <V>
  @param map the first {@link Map}.
  @param other the second {@link Map}.