> (walk (make-ctx expand-defines-walker)
        system-lexenv
        '(lambda ()
           (define foo 42)
           (define square (lambda ()
                            (* foo foo)))
           (list foo square (square)))
        #f)
{lambda ()
  {let {{square #f} {foo #f}}
    {set! square {lambda () {* foo foo}}}
    {set! foo 42}
    {list foo square {square}}}}