prologues := 2;
beginfig(1);
  numeric u;
  u := .4cm;
  drawarrow (0, 0) -- (10u, 0);
  label.bot("V", (10u, 0));
  drawarrow (0, 0) -- (0, 10u);
  label.lft("p", (0, 10u));
  vardef arrowhead expr p =
    save A,u,a,b; pair A,u; path a,b;
    A := point length(p)/2 of p;
    u := unitvector(direction length(p)/2 of p);
    a := A{-u}..(A - ahlength*u rotated 30);
    b := A{-u}..(A - ahlength*u rotated -30);
    (a & reverse(a) & b & reverse(b))--cycle
  enddef;
  pair p, q, r, s;
  p := (4u, 8u);
  q := (9u, 3u);
  s := p shifted -(2u, 0);
  r := q shifted -(0, 1.5u);
  dotlabel.lft("1", s);
  drawarrow s .. p;
  dotlabel.rt("2", p);
  drawarrow p .. controls (((p + q)/2) shifted -(.5u, .5u)) ..  q;
  dotlabel.rt("3", q);
  drawarrow r .. controls (((s + r)/2) shifted -(.5u, .5u)) ..  s;
  dotlabel.bot("4", r);
  drawarrow q .. r;
  currentpicture := currentpicture scaled 2;
  currentpicture := currentpicture shifted -llcorner currentpicture;
endfig;
end;