  Program execution monitoring consists of checking whole executions for given
properties in order to collect global run-time information.
  Monitoring is very useful to maintain programs. However, application
developers face the following dilemma: either they use existing tools which
never exactly fit their needs, or they invest a lot of effort to implement
monitoring code.
  In this article we argue that, when an event-oriented tracer exists, the
compiler developers can enable the application developers to easily code their
own, relevant, monitors which will run efficiently.
  We propose a high-level operator, called foldt, which operates on execution
traces. One of the key advantages of our approach is that it allows a clean
separation of concerns; the definition of monitors is neither intertwined in
the user source code nor in the language compiler.
  We give a number of applications of the foldt operator to compute monitors
for Mercury program executions: execution profiles, graphical abstract views,
and two test coverage measurements. Each example is implemented by a few simple
lines of Mercury.
  Detailed measurements show acceptable performance of the basic mechanism of
foldt for executions of several millions of execution events.
