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

% --- end of displayed preamble in the book ---
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
numeric u;
u=1cm;
z0=origin;z1-z0=(2u,0);
z2-z1=z3-z0=(0,u);
draw z0--z1--z2--z3--cycle;
endfig;
end;