let pretty fmt v = match !v with
| Vstart _ -> Format.fprintf fmt "<start>"
| VfctIn _ -> Format.fprintf fmt "<fctIn>"
| VfctOut _ -> Format.fprintf fmt "<fctOut>"
| Vexit _ -> Format.fprintf fmt "<exit>"
| VblkIn s -> Format.fprintf fmt "<blkIn-%d>" s.sid
| VblkOut s -> Format.fprintf fmt "<blkOut-%d>" s.sid
| _ -> Format.fprintf fmt "<stmt-%d>" (node_type_id !v)