docs: @Pure
  	public PointGroup getGroupAt(int index) {
  		final int count = getGroupCount();
  		if (index < 0) {
  			throw new IndexOutOfBoundsException(index + "<0"); //$NON-NLS-1$
  		}
  		if (index >= count) {
  			throw new IndexOutOfBoundsException(index + ">=" + count); //$NON-NLS-1$
  		}
  		return new PointGroup(index);
  	}