Imagine a robot that, for whatever reason, is performing a sequence
of actions according to some program, and is determined
to complete this execution, no matter what exogenous events
occur in the world while it is executing this program. 
An example of this setting, which we treat in this paper, is a robot 
executing a program to build a tower of blocks in an environment
inhabited by a (sometimes) malicious agent who might arbitrarily move 
some block when the robot is not looking. The robot is equipped with 
sensors, so it can observe when the real world fails to conform to its 
internal representation of what the world would be like in the absence of
malicious agents.  We will understand the execution monitor as a mechanism
that gets output from sensors, compares sensor measurements with
its internal model and, if necessary, invokes a recovery procedure
to make things right again. Our purpose in this paper is to provide
a situation calculus-based, purely logical account of such an
execution monitor, together with a Prolog implementation for it.
To illustrate the theory and implementation, we consider a standard blocks
world as an environment in which a robot is executing a Golog program to
build a suitable tower.