% --- start of displayed preamble in the book ---

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