code: @Nonnull
    public AbstractHasTopLevelRules addRule (@Nonnegative final int nIndex, @Nonnull final ICSSTopLevelRule aRule)
    {
      ValueEnforcer.isGE0 (nIndex, "Index");
      ValueEnforcer.notNull (aRule, "Rule");
  
      if (nIndex >= getRuleCount ())
        m_aRules.add (aRule);
      else
        m_aRules.add (nIndex, aRule);
      return this;
    }
docs: Add a new top-level rule at the specified index. This method only considers
  top-level rules and not <code>@import</code> and <code>@namespace</code>
  rules!
  
  @param nIndex
  The index where the top-level rule should be added. Must be &ge; 0.
  @param aRule
  The rule to be added. May not be <code>null</code>.
  @return this