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arxiv: 2211.10540 · v5 · submitted 2022-11-19 · 💻 cs.FL

Waiting Nets: State Classes and Taxonomy

Pith reviewed 2026-05-24 11:09 UTC · model grok-4.3

classification 💻 cs.FL
keywords waiting netstime Petri netsstate class graphreachabilitycoverabilitytimed language equivalenceexpressivenessbounded nets
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The pith

Bounded waiting nets admit a finite state class graph that decides reachability and coverability while generating strictly more timed languages than time Petri nets.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Waiting nets modify time Petri nets so that a transition can begin measuring time as soon as any of its input places hold tokens, rather than only when the full preset is marked. The semantics also permit firing after an upper time bound even if some resources are still missing, provided the missing tokens arrive later. For any bounded waiting net the construction produces a finite state class graph that abstracts both control locations and time intervals. Reachability and coverability questions can therefore be answered by inspecting this graph. The same model is shown to recognize timed languages that no time Petri net can produce.

Core claim

Waiting nets decouple time measurement from full transition enabling and relax urgency when resources remain unavailable. For bounded instances the paper constructs a finite state class graph by tracking sets of enabled transitions together with symbolic time intervals that account for the waiting semantics. Inspection of this graph decides reachability and coverability. With respect to timed language equivalence the class of waiting nets strictly contains the class of time Petri nets.

What carries the argument

Finite state class graph obtained by symbolic abstraction of control states and time intervals under the waiting semantics that allow partial presets and delayed firing after upper bounds.

If this is right

  • Reachability is decidable for every bounded waiting net.
  • Coverability is decidable for every bounded waiting net.
  • Waiting nets can generate timed languages outside the expressive power of time Petri nets.
  • The finite-graph construction remains valid even though urgency is relaxed and time may start on incomplete presets.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The waiting semantics may capture systems in which timers run independently of resource acquisition without immediately losing decidability.
  • Waiting nets occupy an intermediate position between time Petri nets and stopwatch extensions that are already known to be undecidable on bounded nets.
  • The expressiveness gap suggests that any complete taxonomy of timed Petri-net variants must place waiting nets strictly above ordinary time Petri nets.

Load-bearing premise

The waiting nets under consideration are bounded, that is, each place can hold only finitely many tokens.

What would settle it

A concrete bounded waiting net whose successive state classes keep producing new time intervals indefinitely, or a timed word accepted by some waiting net but rejected by every time Petri net.

Figures

Figures reproduced from arXiv: 2211.10540 by Lo\"ic H\'elou\"et, Pranay Agrawal.

Figure 1
Figure 1. Figure 1: A TPN a) a Timed Petri net b), and a Waiting net c) Consider the examples of [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: a) Decoupled time and control in a waiting net. b) ... with a timeout transition. The semantics of waiting nets associates clocks to transitions, and lets time elapse if their standard preset is filled. It allows firing of a transition t if the standard and the control preset of t is filled. Definition 3.2. (Enabled, Fully Enabled, Waiting transitions) • A transition t is enabled in marking M.N iff M ≥ ◦ (… view at source ↗
Figure 3
Figure 3. Figure 3: Operational semantics of Waiting Nets A timed move from a configuration (M.N, v) lets d ∈ R ≥0 time units elapse, but leaves the marking unchanged. We adopt an urgent semantics that considers differently fully enabled transitions and waiting transitions. First of all, elapsing of d time units from (M.N, v) is allowed only if, for every fully enabled transition t, v(t) + d ≤ β(t). If v(t) + d > β(t) then t … view at source ↗
Figure 4
Figure 4. Figure 4: A Waiting net, its initial domain, an equivalent DB [PITH_FULL_IMAGE:figures/full_fig_p014_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Upper bounds of transitions influence state classe [PITH_FULL_IMAGE:figures/full_fig_p015_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: A non-deterministic state class graph for the wait [PITH_FULL_IMAGE:figures/full_fig_p019_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: A cyclic Waiting net The main idea of this example is to give control alternatively to transition t1 and t2, but measure enabled time for t2 when control allows firing of t1 and enabled time for t1 when control allows firing of t2. The timed word (t1, 1)(t1, 2)(tc3, 3)(t2, 3.2) is a possible behavior of this net. Notice that transition t2 becomes fully enabled after firing of t3, but was enabled from the b… view at source ↗
Figure 8
Figure 8. Figure 8: a) A timed automaton A1 b) An equivalent timed Petri net c) An equivalent TPN in T P Nǫ [PITH_FULL_IMAGE:figures/full_fig_p027_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: a) A waiting net W b) a part of TPN needed to encode L(W) c) An unbounded waiting net The above conditions are needed to ensure that L(N ) = L(W). We can now show that it is impossible to build a net N satisfying all these constraints. Since t1 must fire after t0 , N must contain a subnet of the form shown in figure 9-b, where p is an empty place preventing firing t1 before t0. Notice however that p forbid… view at source ↗
Figure 10
Figure 10. Figure 10: A TPN N2 with non-injective labeling. Corollary 5.11. BT P Ninj <L BW T P Ninj Proof: Inclusion BT P Ninj ≤L BW T P Ninj is straightforward from Definition 3.1. Now this inclusion is strict. Take the example of [PITH_FULL_IMAGE:figures/full_fig_p031_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Relation among net and automata classes, and fron [PITH_FULL_IMAGE:figures/full_fig_p032_11.png] view at source ↗
read the original abstract

In time Petri nets (TPNs), time and control are tightly connected: time measurement for a transition starts only when all resources needed to fire it are available. Further, upper bounds on duration of enabledness can force transitions to fire (this is called urgency). For many systems, one wants to decouple control and time, i.e. start measuring time as soon as a part of the preset of a transition is filled, and fire it after some delay \underline{and} when all needed resources are available. This paper considers an extension of TPN called waiting nets that dissociates time measurement and control. Their semantics allows time measurement to start with incomplete presets, and can ignore urgency when upper bounds of intervals are reached but all resources needed to fire are not yet available. Firing of a transition is then allowed as soon as missing resources are available. It is known that extending bounded TPNs with stopwatches leads to undecidability. Our extension is weaker, and we show how to compute a finite state class graph for bounded waiting nets, yielding decidability of reachability and coverability. We then compare expressiveness of waiting nets with that of other models w.r.t. timed language equivalence, and show that they are strictly more expressive than TPNs.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The paper claims that waiting nets, an extension of time Petri nets allowing time measurement to start with incomplete presets and ignoring urgency in certain cases, admit a finite state class graph when bounded, leading to decidability of reachability and coverability. It also claims strict expressiveness superiority over TPNs w.r.t. timed language equivalence.

Significance. If substantiated, the results would offer a decidable yet more expressive alternative to TPNs for modeling timed systems, positioned between TPNs and undecidable stopwatch TPNs. The state class graph would enable practical verification.

major comments (1)
  1. [Abstract] The finite state class graph construction for bounded waiting nets and the proof of strict timed-language expressiveness over TPNs are asserted without any construction details, proof sketches, or counter-examples provided in the visible text. These are load-bearing for the central claims of decidability and the taxonomy.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their review. The major comment concerns the level of detail in the abstract; we clarify that the full manuscript contains the requested constructions and proofs, as summarized below.

read point-by-point responses
  1. Referee: [Abstract] The finite state class graph construction for bounded waiting nets and the proof of strict timed-language expressiveness over TPNs are asserted without any construction details, proof sketches, or counter-examples provided in the visible text. These are load-bearing for the central claims of decidability and the taxonomy.

    Authors: The abstract is a high-level summary; the finite state class graph construction (including the algorithm for computing classes and the proof that it remains finite for bounded waiting nets) is given in full in Section 3, with the decidability corollaries following directly. The strict expressiveness result over TPNs (via timed language equivalence) is established in Section 4 by exhibiting a concrete timed language accepted by a waiting net but not by any TPN, together with the formal proof. We can add one-sentence pointers to these sections in a revised abstract if the editor prefers. revision: partial

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper defines waiting nets via new semantics that decouple time measurement from control, then constructs a finite state class graph explicitly for the bounded case. Decidability of reachability/coverability follows as a direct consequence of that construction. The strict expressiveness claim over TPNs is obtained by exhibiting timed languages that waiting nets can realize but TPNs cannot. No equations reduce a claimed result to a fitted parameter or to a self-citation chain; the central arguments rest on the freshly stated operational rules and the standard state-class technique adapted to the new firing condition. The known undecidability of stopwatch extensions is invoked only as contrast and is not load-bearing for the positive results.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The central claim rests on the newly introduced semantics of waiting nets and the explicit restriction to bounded nets. No numerical free parameters appear. The model itself is the main invented entity.

axioms (1)
  • domain assumption The waiting net is bounded
    The finite state-class graph and decidability statements are given only under this restriction.
invented entities (1)
  • Waiting net no independent evidence
    purpose: Timed Petri-net variant that decouples time measurement from full preset availability
    New model defined in the paper; no independent evidence supplied beyond the definition itself.

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Reference graph

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    N0), v′ 0(t) = min( v(t) + d1, β (t)) if t ∈ enabled(M0) \ FullyEnabled(M0) and v′ 0(t) = ⊥ otherwise

    where v′ 0(t) = v0(t) + d1 if t ∈ FullyEnabled(M0. N0), v′ 0(t) = min( v(t) + d1, β (t)) if t ∈ enabled(M0) \ FullyEnabled(M0) and v′ 0(t) = ⊥ otherwise. The valuation v1 is computed according to timed moves rules, which means that v1(t) =      0 if t ∈↑ enabled(M1, t 1) , v ′ 0(t) if t ∈ Enabled(M0) ∩ Enabled(M1) ⊥ otherwise Now, since ρ = ( M0.N 0,...