trace: skip at Redo port succeeds only at second attempt

Hi,

I find that in the tracer, consistently, issuing the skip command at a
Redo port will first just print the same line again. Issuing skip a
second time then shows the expected behavior:

ke@apis:~/Dropbox/prolog/test$ cat skip2.pl
a(X) :-
  b(X).

b(2).
b(3).
ke@apis:~/Dropbox/prolog/test$ swipl -l skip2
% /home/ke/Dropbox/prolog/test/skip2.pl compiled 0.00 sec, 4 clauses
Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 6.4.1)
Copyright (c) 1990-2013 University of Amsterdam, VU Amsterdam
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use ?- help(Topic). or ?- apropos(Word).

?- trace, a(X).
   Call: (7) a(_G1834) ? creep
   Call: (8) b(_G1834) ? skip
   Exit: (8) b(2) ? creep
   Exit: (7) a(2) ? creep
X = 2 ;
   Redo: (8) b(_G1834) ? skip
   Redo: (8) b(_G1834) ? skip
   Exit: (8) b(3) ? creep
   Exit: (7) a(3) ? creep
X = 3.

[trace]  ?-

Is this a bug?

Best,
Kilian