draw(z80--z86);
draw(z90--z96);
draw(z100--z106);
-draw(z110--z116);
drawarrow(z15--z65);
drawarrow(z24--z74);
drawarrow(z33--z83);
label.top(btex $x_1$ etex,z26);
label.top(btex $x_2$ etex,z36);
label.top(btex \dots etex,z66);
-label.top(btex $x_{n-2}$ etex,z106);
-label.top(btex $x_{n-1}$ etex,z116);
+label.top(btex $x_{n-2}$ etex,z96);
+label.top(btex $x_{n-1}$ etex,z106);
endfig;