The ice-cream teachpack defines two things:

  - an image of an empty ice cream cone called empty-cone.

  - a function 

      ;; center-pinhole : image -> image

    that moves the pinhole of an image to the image's center.  This is
    necessary for the functions that add scoops.  These functions are
    easiest to write if the pinhole of the cone image is at the
    center.  By default, the image produced by this function will
    leave the pinhole at that of the first argument to overlay/xy
    (used to put the scoop on the cone).  Use center-pinhole to make
    sure the cone image has its pinhole centered before adding another
    scoop to it.