code: @Nonnull
    public AbstractHasTopLevelRules addRule (@Nonnull final ICSSTopLevelRule aRule)
    {
      ValueEnforcer.notNull (aRule, "Rule");
  
      m_aRules.add (aRule);
      return this;
    }
docs: Add a new top-level rule at the end. This method only considers top-level
  rules and not <code>@import</code> and <code>@namespace</code> rules!
  
  @param aRule
  The rule to be added. May not be <code>null</code>.
  @return this