fill: procedure (x, y, fill_color) recursive; /* 12 May 2010 */
   declare (x, y) fixed binary;
   declare fill_color bit (24) aligned;

   if x <= lbound(image, 2) | x >= hbound(image, 2) then return;
   if y <= lbound(image, 1) | y >= hbound(image, 1) then return;

   pixel_color = image (x,y); /* Obtain the color of the current pixel. */
   if pixel_color ^= area_color then return;
      /* the pixel has already been filled with fill_color, */
      /* or we are not within the area to be filled.        */
   image(x, y) = fill_color; /* color the desired area. */

   pixel_color = image (x,y-1); /* Obtain the color of the pixel to the north. */
   if pixel_color = area_color then call fill (x, y-1, fill_color);

   pixel_color = image (x-1,y); /* Obtain the color of the pixel to the west. */
   if pixel_color = area_color then call fill (x-1, y, fill_color);

   pixel_color = image (x+1,y); /* Obtain the color of the pixel to the east. */
   if pixel_color = area_color then call fill (x+1, y, fill_color);

   pixel_color = image (x,y+1); /* Obtain the color of the pixel to the south. */
   if pixel_color = area_color then call fill (x, y+1, fill_color);

end fill;