code: @Nonnull
    public final HCHead addJSAt (@Nonnegative final int nIndex, @Nonnull final IHCNode aJS)
    {
      ValueEnforcer.notNull (aJS, "JS");
      if (!HCJSNodeDetector.isJSNode (aJS))
        throw new IllegalArgumentException (aJS + " is not a valid JS node!");
      m_aJS.add (nIndex, aJS);
      return this;
    }
docs: Append some JavaScript code at the specified index
  
  @param nIndex
  The index where the JS should be added (counting only JS elements)
  @param aJS
  The JS to be added. May not be <code>null</code>.
  @return this