% --- start of displayed preamble in the book --- def square=(unitsquare shifted (-.5,-.5) scaled 3mm) enddef; def circle=(fullcircle scaled 3mm) enddef; % --- end of displayed preamble in the book --- defaultfont:="ptmr8r"; warningcheck:=0; beginfig(1) for i=0 upto 4: for j=0 upto 4: if odd(i+j): draw square shifted (i*6mm,j*6mm); else:fill circle shifted (i*6mm,j*6mm); fi endfor; endfor; endfig; end;