If there is only one edge it doesn't draw an arrow

[jimhefferon]

If you run this it draws the edge but not the arrow. Uncomment the superfluous edge to get an arrow.

picture pic;
int picnum = 34;
unitsize(pic,1pt);

// define nodes
node q0=ncircle("$q_0$"); 
node q1=ncircle("$q_0$"); 

// calculate nodes position
real u=1.5cm;  // horizontal  
real v=1.0*u;  // vertical
hlayout(1*u, q0, q1);

// draw edges
draw(pic,
     //     (q0--q1).l("a"),
     (q0--q1).l("a")
     );

// draw nodes after edges
draw(pic, q0, q1);

shipout(format(OUTPUT_FN,picnum),pic,format="pdf");