picture FE, FEV, E, EV, M, N, U, V;
FE = thelabel("$f^*E$", (x0, y0));
M = thelabel("$M$", (x0, y1));
E = thelabel("$E$", (x1, y0));
N = thelabel("$N$", (x1, y1));
FEV = thelabel("$f^*E_V$", (x2, y2));
U = thelabel("$U$", (x2, y3));
EV = thelabel("$E_V$", (x3, y2));
V = thelabel("$V$", (x3, y3));
forsuffixes @=E, M, N, U, V, EV, FE, FEV: draw @; endfor
ahangle := 20;
vardef connect(expr a, b) =
save line; path line; interim bboxmargin := 3;
line = center a .. center b cutbefore bbox a cutafter bbox b;
cutdraw line withpen pencircle scaled 4 withcolor background;
drawarrow line
enddef;