[plt-scheme] plot/extend (zgrid ....) not consistent with documentation

The zgrid procedure is documented as

(zgrid f xs ys) → (listof (listof real?))
   f : (real? real? . -> . real?)
   xs : (listof real?)
   ys : (listof real?)
Given a function that consumes x and y to produce z, a list of xs, and  
a list of ys, produces a list of z column values.
But errors out indicating the procedure takes 4 parameters
It is defined in renderer-helpers.ss as
   ; zgrid : (number number -> number) listof-number listof-number ->  
listof-listof number
   (define (zgrid func x-vals y-vals samples)
     (map (lambda (x) (map (lambda (y) (func x y)) y-vals)) x-vals))

The 'samples' parameter is not referenced in the body.

How should the definition or use be corrected?

Thanks
-------------------------------
Lawrence J. Winkler, M.S., J.D.
5306 Loruth Ter
Madison Wisconsin 53711
Phone: 608-274-0414
Mobile: 608-345-0627
Email: winkllj at acm.org
--------------------------------



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