% --- start of displayed preamble in the book ---
input expressg
% --- end of displayed preamble in the book ---
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
z1=(0,0);
drawLEVENT(1, 2cm, 1cm)(btex levent etex);
z2=(0,-2cm);
drawGEVENT(2, 2cm, 1cm)(btex gevent etex);
endfig;
end;