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•    violation of one of the fundamentals of Petri nets, i.e. principle of locality of the decision on the transition firing; i.e. token decisions are local; however time is global for specific transition

•    there are particular timing conditions forcing the firing of a transition and hidden mechanism of arrival times for places

•    these are ad hoc extensions of PN model with little attention to assess their mutual relationship

Example 2: Time conditions are attached to transitions

P1 - place representing customer's reąuest to buy the perishable item P2 - place representing the availability of a perishable item (a bag of apples) P3 - place representing availability of other resources (sugar, preservatives) to make a jam

Tl - transition representing an action of'selling of the item to the customer'; this transition may fire if apples are ripe but not rotten; the firing time of this transition is:

I₁=[time(P2)+t_lmin,time(P2)+t_lmax]

T2 - transition representing an action of 'trashing of a rotten item'; the firing time of this transition is

I₂=[tirne(P2)+t_2min,+cq

T3 - transition representing an action of'making jam'; the firmę transition is

I₃⁼[time(P2)+t_3min,time(P2)+tą_niax] {ujm pĆMiUżłkć flćfriSU&bCtt /'few

/ \ P2

A

P3

cf UP i Pr

	\
/	\
. r	c
	OT

T3

s

l¹!