A simulation, in filu, is the running kernel that is simulating a world. It is the go routine that is receiving ticks from the clock, taking input from the actors and applying it to the simulated world, then calculating the world state of the next frame and sending it back out to the actors. It is the game loop.
A simulation has a life cycle. It begins as an UnstartedSimulation. An unstarted simulation has only one behavior, it can Begin.
Once the simulation has begun, it is a RunningSimulation. A running simulation can connect and remove actors. It can also halt which stops the simulation.
A HaltedSimulation has no defined behavior yet. But it could perhaps contain stats and analytics. Maybe all the Actors that remained connected through the Halt.
rpg2d is an implementation of the Simulation interface and life cycle.
A journal to store work log of entries and ideas. Reading the latest entry will give some context to what's currently being worked on, what's working, what's not working.