error: Invalid call to disp.  Correct usage is:

 -- disp (X)
 -- STR = disp (X)
error: called from
    print_usage at line 91 column 5
    ex2 at line 2 column 1

1 -> 3
1 -> 2
3 -> 2
1 -> 3
2 -> 1
2 -> 3
1 -> 3

1 -> 3
1 -> 2
3 -> 2
1 -> 3
2 -> 1
2 -> 3
1 -> 3