Syntax: SURFACE type z1 z2
or      SURFACE type z1 z2 WORD WORD

   Draw a wire-frame surface of the current IMAGE. If the WORDs are omitted
a line in the surface will be drawn for each row and column of the image;
if the WORDs are present they will be taken to be the x- and y- coordinates
of the desired lines, and SM will interpolate in the image to determine
the corresponding values (see also the hundreds digit of type, below).

   The last digit of TYPE is used to determine which surface to draw:
        0       no hidden line removal
        1       draw top surface
        2       draw bottom surface
        3       draw both top and bottom surfaces

   If TYPEs tens digit is set, SURFACE will use the current limits (as
set with LIMITS) rather than autoscaling them from the data.

   If TYPEs hundreds digit is set, the two WORDs are taken to be the
x- and y- coordinates corresponding to the rows and columns of the IMAGE,
but no interpolation is done.

   Z1 and Z2 are the limits used for the z-axis; you might want to set them
with MINMAX.

   See also VIEWPOINT.

   There are some useful macros in the file `surfaces'; say "load surfaces"
to read them.