How to dynamically define a menu item - what's in square brackets?

It's not clear about some fundamental syntax here. define-key

takes a set of inputs, one of which is inside square brackets. What is this construction? How can I dynamically generate what is included in the square brackets?

In a simple case, I can display a one-way menu like this:

(flet ((ok (&optional p1 &rest args) t))
  (setq menu-1 (make-sparse-keymap "Title"))
  (define-key menu-1 [menu-1-ok-event]
    `(menu-item "OK"
                ok 
                :keys ""
                :visible t
                :enable t))
  (x-popup-menu t menu-1))

      

        
        
        
      

    

I can insert additional menu items like:

(flet ((ok (&optional p1 &rest args) t))
  (setq menu-1 (make-sparse-keymap "Title"))
  (define-key menu-1 [menu-1-event-ok]
    `(menu-item "OK"
                ok 
                :keys ""
                :visible t
                :enable t))
  (define-key menu-1 [menu-1-event-1]
    `(menu-item "This is line 1"
                nil
                :keys ""
                :visible t
                :enable t))
  (x-popup-menu t menu-1))

      

        
        
        
      

    

But what if I want to dynamically generate the thing inside the square brackets? What if I want something like this:

  (while (< n 5)
   (define-key menu-1 [(dynamic-thing n)]
    `(menu-item (format "This is line %d" n)
                nil
                :keys ""
                :visible t
                :enable t)))

      

        
        
        
      

    

I tried

   (define-key menu-1 [(intern (format "menu-1-event-%d" n))]
      ...

      

        
        
        
      

    

.. but it didn't work. The result is always "trainee". ???

What are square curly braces? The syntax is unfamiliar to me.