code: @Nonnull
    public CascadingStyleSheet addImportRule (@Nonnegative final int nIndex, @Nonnull final CSSImportRule aImportRule)
    {
      ValueEnforcer.isGE0 (nIndex, "Index");
      ValueEnforcer.notNull (aImportRule, "ImportRule");
  
      if (nIndex >= getImportRuleCount ())
        m_aImportRules.add (aImportRule);
      else
        m_aImportRules.add (nIndex, aImportRule);
      return this;
    }
docs: Add a new <code>@import</code> rule at a specified index of the
  <code>@import</code> rule list.
  
  @param nIndex
  The index where the rule should be added. Must be &ge; 0.
  @param aImportRule
  The import rule to add. May not be <code>null</code>.
  @return this
  @throws ArrayIndexOutOfBoundsException
  if the index is invalid