Why removing N variable in equationsToTransitions?

[SergeStinckwich]

Why do we need to remove N from the events in equationsToTransitions method:

equationsToTransitions

	"This is only working for status ODE equations, not general ODE equations"

	| events |
	events := self generateEvents.

	"Remove N variable in events rate. Why?"
	events do: [ :e | 
		e fromStatus = #empty ifTrue: [ 
			e rate: (e rate removeVariable: (KEVariable new symbol: #N)) ] ].

	events do: [ :each | 
		self
			addTransitionFrom: { (#status -> each fromStatus) }
			to: { (#status -> each toStatus) }
			probability: each rate ].

	"Why initializing equations at the end?"
	equations := OrderedCollection new

[btmanh]

Because in the equation of S, dS/dt = muN - lambdaS, we have the transition: empty — mu —> S
however the rate extract from the equation is muN, and we store only per capita transition rate, so to obtain the rate mu, we remove N.

[SergeStinckwich]

Maybe a correct way to do it, is to store only per capita transition rate when we generate the events (in method generateEvents), so don't have to deal with that later.

[btmanh]

Yes, i agree. And it seems that in the generateEvents method, we store per capita rate for other events, except empty-->S.

[SergeStinckwich]

Why there is a special treatment for empty -> S ?