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

% --- end of displayed preamble in the book ---
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
draw (0,0)..(1cm,0)..(2cm,1cm)..(3cm,3cm);
draw (0,0)--(3cm,3cm);
picture p;
p=currentpicture;
for i within p:
  if stroked i:
    if length(pathpart i)>2:
      draw pathpart i withcolor blue;
      fi
    fi
endfor;
endfig;
end;