% --- start of displayed preamble in the book ---
input exteps
% --- end of displayed preamble in the book ---
% graphic converted to gray in book using 'color2gray'
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
begineps "gradient.eps";
  base := (25,25);
  clipping:=true;
  grid:=true;

  epsdrawdot(10pct,40pct)
    withpen pencircle scaled 10pct
    withcolor blue;
  epsdrawdot(60pct,20pct)
    withpen pencircle scaled 10pct
    withcolor red;
endeps;
draw origin withpen pencircle scaled 30
            withcolor blue+green;
endfig;
end;