Simulation Collection Version 2.1 Available

I just released Version 2.1 of the Simulation Collection to PLaneT. This fixes a bug in the variable monitors and updates the interacting particles example (interactions.ss) to illustrate their use in graphical simulations. If anyone with more experience with graphics programming with MrED would look it over and make suggestions on improving it, I would appreciate it.
Variables are used for automatic data collection in the Simulation Collection. [Note that these variables are not Scheme identifiers with values associates with them, but a separate data type and associated semantics.] Variable monitors allow arbitrary code to be attached to getting or setting a variable's value. This is especially useful in adding graphics to a simulation. In the particle interactions example, one is used to update the progress bar.
The particle interactions example also uses a simulation monitor that is executed each time the simulation clock is advanced. This provides the main simulation visualization.
This weekend I plan on adding process monitors that are executed whenever a process event is executed. This will be useful for updating graphics based on the (change in) state of a process. I will also update the reference manual with a section on Monitors.