Hi, swiprolog on Deibian ppc, current versions, generates the following error message when triggering the trace: Sergio --- Welcome to SWI-Prolog (Version 3.3.0) Copyright (c) 1993-1999 University of Amsterdam. All rights reserved. For help, use ?- help(Topic). or ?- apropos(Word). 1 ?- trace. ERROR: trace/0: Caught signal 11 (segv) [PROLOG SYSTEM ERROR: Recursively received fatal signal 11 PROLOG STACK: #