weak until
TRANSCRIPT
Weak until, release and positive normal form
Nadeem Qasmi
Sadain Iqrar
Abdur Rehman Abbasi
Syntax of LTL (Linear temporal logic) formula
• LTL formulae over the set AP of atomic proposition are formed according to the following grammar*
where a ∈ AP.
* Backus Naur form (BNF)
Positive Normal Form of LTL formula
• Also called as Canonical Form
• BNF to positive normal form transformation
• Rules:
• Negation can only occur adjacent to Atomic prepositions
• For every operator, it’s dual operator is needed to be incorporated
Negation Rule
• This is done by successively “pushing” negations “inside” the formula at hand
¬true false
¬false true
¬¬ϕ ϕ
¬(ϕ ∧ ψ) ¬ϕ∨¬ψ
¬ Oϕ O¬ϕ
These rewrite rules are lifted to the derived operators as follows:
¬(ϕ ∨ ψ) ¬ϕ ∧¬ψ and ¬ ♦ ϕ ¬ϕ and ¬ ϕ ♦¬ϕ.
Negation Rule
• Example:
• Consider the LTL formula ¬ (( a Ub ) ∨ Oc). This formula is not in PNF, but can be transformed into an equivalent LTL formula in weak-until PNF as follows:
¬ (( aUb) ∨ Oc)
≡ ♦¬(( aUb ) ∨ Oc)
≡ ♦(¬ (aUb) ∧ ¬ Oc)
≡ ♦((a∧¬b) W (¬a∧¬b) ∧ O¬c)
Dual Operator Rule
• For every operator it’s dual must be incorporated
• For BNF operators Dual operators are
ϕ ::= true | a | ϕ1 ^ ϕ2 |¬ϕ | Oϕ | ϕ1 Uϕ2
• Duals:
false = ¬ true,
true = ¬ false,
¬ (ϕ ∨ ψ) ≡ ¬ ϕ∧¬ ψ (Disjunction)
¬ (ϕ∧ ψ) ≡ ¬ ϕ ∨¬ ψ (Conjunction)
¬ O ϕ ≡ O¬ ϕ, (Dual of itself)
Weak Until as Dual of Until
• Consider the “Until” Operator
¬ (ϕUψ) ≡ ((ϕ∧¬ψ) U (¬ ϕ ∧¬ ψ)) ∨ ( ϕ ∧¬ ψ)
• This observation provides the motivation to introduce the operator W (called weak until or unless) as the dual of U. It is defined by:
Φ W ψ ≡ (ϕU ψ) ∨ ϕ.
• Until and Weak-Until/Unless Duality
¬ ( ϕU ψ) ≡ (ϕ∧¬ ψ) W (¬ ϕ∧¬ ψ)
¬ ( ϕWψ) ≡ (ϕ∧¬ ψ) U (¬ ϕ∧¬ ψ)
Weak Until
• It is interesting to observe that W and U satisfy the same expansion law:
Lemma 5.19
• Weak-Until is the Greatest Solution of the Expansion Law
• The formulation “greatest LT property with the indicated condition (*) is to be understood in the following sense:
(1) P ⊇ Words ( ϕ W ψ) satisfies (*).
(2) Words ( ϕ W ψ) ⊇ P for all LT properties P satisfying condition (*).
Lemma 5.19
Positive Normal Form for LTL (Weak-Until PNF)
• For a ∈ AP, the set of LTL formulae in weak-until positive normal form (weak-until PNF, for short, or simply PNF) is given by:
Theorem 5.22.
• Existence of Equivalent Weak-Until PNF Formulae
• For each LTL formula there exists an equivalent LTL formula in weak-until PNF.
¬(ϕU ψ) ≡ (¬ψ)W(¬ϕ ∧¬ψ)
Release Operator
• To avoid the exponential blowup in transforming an LTL formula in PNF, another temporal modality is used that is dual to the until operator: the so-called binary release-operator, denoted R . It is defined by
Φ R ψ ≡ ¬ (¬ ϕ U¬ ψ).
• Formula ϕ R ψ holds for a word if ψ always holds, a requirement that is released as soon as ϕ becomes valid.
• The weak-until and the until operator are obtained by:
Φ W ψ ≡ (¬ ϕ ∨ ψ ) R (ϕ ∨ ψ) , ϕ U ψ ≡ ¬ (¬ ϕR ¬ ψ).
Φ R ψ ≡ (¬ ϕ ∧ ψ ) W ( ϕ ∧ ψ ).
Release Operator
• The expansion law for release reads as follows:
Φ R ψ ≡ ψ ∧ (ϕ ∨ O( ϕ R ψ))
• For a ∈ AP, LTL formulae in release positive normal form (release PNF, or simply PNF) are given by
Question ?