Avoiding unification equality / from variable to atom

Hi all,

Here I present a simple case which describes my problem:

| ?-  memberchk(rel(_,A,_),[rel(de,B,C),rel(para,C,D)]).
B = A

yes
| ?- 

I'd like this memberchk/2 call to answer "No", stated that the
content on the list argument is unknown and as such the A \== B 
constraint it's not available.

Is there a way to avoid unification/equality between variables?
Or a translator from variable to atom (uppercase to lowercase)?

Any hints?


Greetings,
Peter