code: @Nonnull
    public CascadingStyleSheet addNamespaceRule (@Nonnull final CSSNamespaceRule aNamespaceRule)
    {
      ValueEnforcer.notNull (aNamespaceRule, "NamespaceRule");
  
      m_aNamespaceRules.add (aNamespaceRule);
      return this;
    }
docs: Add a new <code>@namespace</code> rule at the end of the
  <code>@namespace</code> rule list.
  
  @param aNamespaceRule
  The namespace rule to be added. May not be <code>null</code>.
  @return this