The content discusses the proof theory of semi-substructural logics, which are logical systems that relax some of the structural rules of intuitionistic linear logic. The authors focus on extending the sequent calculi for skew monoidal categories, previously studied by the authors and collaborators, with additive conjunction and disjunction.
The key highlights are:
The authors describe a cut-free sequent calculus for a fragment of non-commutative linear logic consisting of the skew multiplicative unit I, conjunction ⊗, and the additive connectives ∧and ∨. They introduce a congruence relation ⊜ that identifies derivations up-to η-equivalence and permutative conversions.
The authors provide a categorical semantics for this logic in terms of distributive skew monoidal categories, which are skew monoidal categories with binary products and coproducts satisfying a left-distributivity condition.
The authors introduce a focused sequent calculus with tag annotations that provides a sound and complete normalization procedure for the original sequent calculus. The focused calculus employs a root-first proof search strategy that uses tags to control the non-deterministic choices in the application of non-invertible rules.
The authors discuss extensions of the logic with additive units, a form of skew exchange, and linear implication. They argue that the normalization technique is modular and potentially scalable to other richer substructural logics.
The sequent calculi and the normalization procedure have been fully formalized in the Agda proof assistant and are available online.
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