% --- start of displayed preamble in the book --- % --- end of displayed preamble in the book --- defaultfont:="ptmr8r"; warningcheck:=0; beginfig(1) numeric r,pr,x,rmin,rmax,ur,ux; rmin=1.9; rmax=3; ur=5cm; ux=6cm; r:=rmin; pr:=(rmax-rmin)/250; for i=1 upto 250: x:=.3; for j=1 upto 50: x:=(1+r)*x-r*x*x; endfor; for j=1 upto 50: x:=(1+r)*x-r*x*x; draw (r,x) xscaled ur yscaled ux; endfor; r:=r+pr; endfor; endfig; end;