SWI Prolog forgets identity of query variables in CHR ???

Hi SWIPL team,

I observed strange problem that the identity of variables in CHR queries are sometimes forgotten when the resylt is printed out.
I could isolate the problem into a one rule program.
For the two test queries, no rule is applied;
the first one behaves as you would expect, but in the second one, my query variables gets renamed and thus the semantics of the answer is wrong.

If I have a program with no rules, no misbehaviour is observed

(running on Mac OS 10.9.1)

best wishes

     Henning


The program:

:- use_module(library(chr)).
:- chr_constraint c/2, d/2.
c(X,cow) <=> d(X,pig).

Queries:
$ swipl
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 6.6.1-DIRTY)
....

?- [program].
%  library(chr) compiled into chr 0.51 sec, 6,504 clauses
% program compiled 0.51 sec, 6,551 clauses
true.

?- c(X,duck).
c(X,duck)
true .
c(X,duck)

?- c(X,Y).
c(_G30729,_G30730)
true .
c(_G30729,_G30730)





Henning Christiansen
Professor of  Computer Science, PhD
Leader of Research group PLIS: Programming, Logic and Intelligent Systems
Department of Communication, Business and Information Technologies
Roskilde University
P.O.Box 260, DK-4000 Roskilde, DENMARK
http://www.ruc.dk/~henning

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