code: @Nonnull
    public CascadingStyleSheet addNamespaceRule (@Nonnegative final int nIndex,
                                                 @Nonnull final CSSNamespaceRule aNamespaceRule)
    {
      ValueEnforcer.isGE0 (nIndex, "Index");
      ValueEnforcer.notNull (aNamespaceRule, "NamespaceRule");
  
      if (nIndex >= getNamespaceRuleCount ())
        m_aNamespaceRules.add (aNamespaceRule);
      else
        m_aNamespaceRules.add (nIndex, aNamespaceRule);
      return this;
    }
docs: Add a new <code>@namespace</code> rule at the specified index of the
  <code>@namespace</code> rule list.
  
  @param nIndex
  The index where the rule should be added. Must be &ge; 0.
  @param aNamespaceRule
  The namespace rule to be added. May not be <code>null</code>.
  @return this