
OVER

The OVER operator performs a "transparent" overlay operation.  It 
overlays the result of the EXPR onto the map "mapname".  If 
"mapname doesnt exist, it is created.  Up to 4 overlays can be done
on each saved map where overlay 1 is represented with red, 2 yellow,
3 blue, and 4 grey.  To preserve the overlay information, the overlays
are assigned the following values: overlay 1 takes the value 1, 2 2,
3 4, and 4 8.  The actual value assigned to a cell is the sum of these
values representing overlays.  Hence, map category 5 represents overlay
1 (value 1) and overlay 3 (value 4).  The color purple would represent 
5 and would visually show the transparent overlay of 1 (red) and 3
(blue).

syntax:   (OVER mapname # EXPR)  where # is one of these: 1 2 3 4

operation:  To overlay soils type 3 thru 5 with geology types 6 and 9:
  (NAME demo1 (OVER RED (GROUP 3-5 (soils) )) )
  (NAME demo2 (OVER demo1 YELLOW (GROUP 6 9 (geology) ))) 

aliases:
    OVER     over     overlay
