  This paper specifies an observational semantics and gives an original
presentation of the Byrd box model. The approach accounts for the semantics of
Prolog tracers independently of a particular Prolog implementation. Prolog
traces are, in general, considered as rather obscure and difficult to use. The
proposed formal presentation of its trace constitutes a simple and pedagogical
approach for teaching Prolog or for implementing Prolog tracers. It is a form
of declarative specification for the tracers. The trace model introduced here
is only one example to illustrate general problems relating to tracers and
observing processes. Observing processes know, from observed processes, only
their traces. The issue is then to be able to reconstitute, by the sole
analysis of the trace, part of the behaviour of the observed process, and if
possible, without any loss of information. As a matter of fact, our approach
highlights qualities of the Prolog resolution box model which made its success,
but also its insufficiencies.
