content: 
  -- It corresponds to model MachRep1 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.
  --
  -- Two machines, which sometimes break down.
  -- Up time is exponentially distributed with mean 1.0, and repair time is
  -- exponentially distributed with mean 0.5. There are two repairpersons,
  -- so the two machines can be repaired simultaneously if they are down
  -- at the same time.
  --
  -- Output is long-run proportion of up time. Should get value of about
  -- 0.66.
  
  import Control.Monad.Trans
  
  import Simulation.Aivika
  
  meanUpTime = 1.0
  meanRepairTime = 0.5
  
  specs = Specs { spcStartTime = 0.0,
                  spcStopTime = 1000.0,
                  spcDT = 1.0,
                  spcMethod = RungeKutta4,
                  spcGeneratorType = SimpleGenerator }
          
  model :: Simulation Results
  model =
    do totalUpTime <- newRef 0.0
       
       let machine :: Process ()
           machine =
             do upTime <-
                  randomExponentialProcess meanUpTime
                liftEvent $ 
                  modifyRef totalUpTime (+ upTime)
                repairTime <-
                  randomExponentialProcess meanRepairTime
                machine
  
       runProcessInStartTime machine
       runProcessInStartTime machine
  
       let upTimeProp =
             do x <- readRef totalUpTime
                y <- liftDynamics time
                return $ x / (2 * y)
  
       return $
         results
         [resultSource
          "upTimeProp"
          "The long-run proportion of up time (~ 0.66)"
          upTimeProp]
    
  main =
    printSimulationResultsInStopTime
    printResultSourceInEnglish
    model specs
  