You'll notice the addition of the Fire subject. The fire subject is
essentially a simulation, its main loop is identical to that of the simulation
subject. The only difference is, that the fire subject adds an atom with
a velocity that puts it on track to collide with the given structure of atoms.
Given that the fire subject is so similar to the simulate subject, it is not
necessary to implement a whole new class for it. Rather, we let Fire
inherit from Simulate. Then we redefine the methods we need to. The two
methods that we redefine are the loadConfiguration method, and the
preLoopConfig method. preLoopConfig was added to Simulate to
allow classes such as Fire to extend its run routine without completely
redefining it.
The end result is that Fire only has the code relevant to adding a firing
atom, it does not have to worry about simulating it. VPRenderer had to be
modified in order to accommodate for the target and arrows, however due to the
nature of the design, none of its code was actually modified, it only had
the relevant parts added.