[racket] Slideshow: change baseline

Hi,

Is it possible in slideshow to change the baseline of a pict?

I have the following code to draw a fraction:

;; Fraction
;; pict? pict? -> pict?
(define (frac a b)
  (vc-append
   a
   (hline (max (pict-width a) (pict-width b)) 20)
   b))

and I want to do some thing like:
(hbl-append (t "x=") (frac (t "1234") (t "5678")))

so that the hline is aligned with the =.

I tried the various procedures like `lift-above-baseline` and such but with
no luck.

Thanks,
Laurent
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