3:ab    # for @set=(c,d); return to 2 

    4:abc  # ...

    4:abd  # ...


##</code><code>##

 
    4:abc    # for @set=(d); return to 3

     5:abcd  # print result; return to 4


# ... later


    4:abd    # for @set=(c); return to 3

     5:abdc  # print result; return to 4