beginfig(4); u=1 cm; z0=(2u,2u); z1=(4u,3u); drawarrow z0..z1 ; z2 =z0+1/3(z0-z1); drawarrow z0..z2; dotlabel.bot(btex $O$ etex, z0); label.top(btex $M$ etex, z1); label.ulft(btex $M'$ etex, z2); z3=(3.2u,1.6u); label.top(btex $k=-\frac{1}{3}$ etex, z3); endfig; end