Consider the classic first example:

A port has 2 jetties each of which can be used for unloading by one boat at a time. Boats arrive at the port periodically and must wait if no jetty is currently free. When a jetty is available, a boat may dock and start to unload. When this activity has been completed, the boat leaves the jetty and sails away. The port authority has a pool of 3 tugs. Two are required for docking: only one when a boat leaves its jetty.

This model would be written in Demos as follows:

class boat=
     {getR(jetties,1); 
        getR(tugs,2);
         hold(2.0);
        putR(tugs,2);

        hold(14.0);

        getR(tugs,1);
         hold(2.0);
        putR(tugs,1); 
      putR(jetties,1);
     }      (***boat***)

    Res(tugs, 3);
    Res(jetties, 2);
    Entity(boat,boat,0.0);
    Entity(boat,boat,1.0);
    Entity(boat,boat,15.0);

    hold(36.0);
    Close;