% --- 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;