It corresponds to model MachRep3 described in document
Introduction to Discrete-Event Simulation and the SimPy Language
[http://heather.cs.ucdavis.edu/~matloff/156/PLN/DESimIntro.pdf].
SimPy is available on [http://simpy.sourceforge.net/].
The model description is as follows.
Variation of models MachRep1, MachRep2. Two machines, but
sometimes break down. Up time is exponentially distributed with mean
1.0, and repair time is exponentially distributed with mean 0.5. In
this example, there is only one repairperson, and she is not summoned
until both machines are down. We find the proportion of up time. It
should come out to about 0.45.