Why N is not defined as a parameter when we define a new model ?

[SergeStinckwich]

Why do we distinguish N from other parameters when we define a model ?

Example :

model := KEModel new.
	model population: (KEPopulation size: 5000).
	SEIRConcern := KEModelPart new.
	SEIRConcern attributes: {(#status -> #(#S #E #I #R))}.
	SEIRConcern addParameters: {#beta . #gamma . #mu . #sigma}.
	SEIRConcern addParameter: #lambda value: 'beta*I/N'.
	SEIRConcern
		addEquations:
			{'S:t=mu*N - lambda*S - mu*S' . 'E:t=lambda*S - sigma*E - mu*E' . 'I:t=sigma*E - gamma*I - mu*I' . 'R:t=gamma*I - mu*R'}.

N is not defined as a parameter like beta, gamma, ...