Traces of program executions are a helpful source of information for automated debugging. They, however, usually give a too low level picture of the executed program. Opium, our extendable trace analyser for Prolog, is connected to a «standard» tracer. Opium is programmable and extendable. It provides a trace query language and abstract views of executions which solve the problems of low-level traces. Opium has shown its capabilities to build abstract tracers and automated debugging facilities. This article describes the trace query mechanism, from the model to its implementation. Characteristic examples are detailed. Extensions written so far on top of the trace query mechanism are listed. Two recent extensions are presented: the abstract tracers for the LO (Linear Objects) and the CHR (Constraint Handling Rules) languages. These two extensions were specified and implemented within a few days. They show how to use Opium for real applications.