add(checkNotNull(element));
      }
      return this;
    }

    @CanIgnoreReturnValue
    public Builder<E> addAll(Iterator<? extends E> elements) {
      checkNotNull(elements); // for GWT
      while (elements.hasNext()) {
        add(checkNotNull(elements.next()));
      }
      return this;
    }

    public abstract ImmutableCollection<E> build();
  }

 * </ul>
 *
 * <h4>Warnings</h4>
 *
 * <ul>
 *   <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
 *       (in a way that affects its {@link Object#equals} behavior) while it is contained in a
 *       collection. Undefined behavior and bugs will result. It's generally best to avoid using
 *       mutable objects as elements at all, as many users may expect your "immutable" object to be

 * </ul>
 *
 * <h4>Warnings</h4>
 *
 * <ul>
 *   <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
 *       (in a way that affects its {@link Object#equals} behavior) while it is contained in a
 *       collection. Undefined behavior and bugs will result. It's generally best to avoid using
 *       mutable objects as elements at all, as many users may expect your "immutable" object to be