Gprolog member/2 error ?!

Hi all,

I found a strange error on my gprolog coding, namely on the source:

pred1([R-C-P|Ls], [R-C-P|Ls1]):-
    memberchk(rel(_,X,Y), C),
    memberchk(rel(_,Y,_), C),
    nth(1,P,Z),
    ...

I get the debug output:

...call pred1...

   6913    3  Call: 
   memberchk(
		rel(_215226,_215227,_215228),
		['conclusao'(_198617),'diagnostico'(_198661),
		 paciente(_198705),nome(_198705,'Rita Ambrosio'),
		 rel(de,_198617,_198661),rel(para,_198617,_198705)
		]
	) ? 
   6913    3  Exit: 
   memberchk(
		rel(de,_198617,_198661),
		['conclusao'(_198617),'diagnostico'(_198661),
		 paciente(_198705),nome(_198705,'Rita Ambrosio'),
		 rel(de,_198617,_198661),rel(para,_198617,_198705)
		]
	) ? 
   6914    3  Call: 
   memberchk(
		rel(_215255,_198661,_215257),
		['conclusao'(_198617),'diagnostico'(_198661),
		 paciente(_198705),nome(_198705,'Rita Ambrosio'),
		 rel(de,_198617,_198661),rel(para,_198617,_198705)
		]
	) ? 
   6914    3  Exit: 
   memberchk(
		rel(de,_198617,_198617),
		['conclusao'(_198617),'diagnostico'(_198617),
		 paciente(_198705),nome(_198705,'Rita Ambrosio'),
		 rel(de,_198617,_198617),rel(para,_198617,_198705)
		]
	) ? 
 + 6915    3  Call: nth(1,[ser(_198749,a:_198617)],_215322) ? 
 ...

 
 Also tried with member/2 but the List is always changed on exit of the 
 secound call to member/2. Notice the rel(de,_198617,_198661) predicate
 is changed to rel(de,_198617,_198617) although member/2 is not stated
 as a destructive process.
 
 Although my rel/3 predicate contains a literal and 2 prolog variables
 I'd like to compare variable instances, as stated on the above source.

 Any help ?
 
 
Greetings,
Peter