code: @Nonnull
    public CascadingStyleSheet addImportRule (@Nonnull final CSSImportRule aImportRule)
    {
      ValueEnforcer.notNull (aImportRule, "ImportRule");
  
      m_aImportRules.add (aImportRule);
      return this;
    }
docs: Add a new <code>@import</code> rule at the end of the <code>@import</code>
  rule list.
  
  @param aImportRule
  The import rule to add. May not be <code>null</code>.
  @return this