IntroductionNon-commutative logical operators have a long tradition [12,22,2,13,16,3], and their proof theoretical properties have been studied in the sequent calculus [7] and in proof nets [8]. Recent research has shown that the sequent calculus is not adequate to deal with very simple forms of non-commutativity [9,10,21]. On the other hand, proof nets are not ideal for dealing with exponentials and additives, which are desirable for getting good computational power. In this paper we show a logical system that joins a simple form of noncommutativity with commutative multiplicatives and exponentials. This is done in the formalism of the calculus of structures [9,10], which overcomes the difficulties encountered in the sequent calculus and in proof nets. Structures are expressions intermediate between formulae and sequents, and in fact they unify those two latter entities into a single one, thereby allowing more control over mutual dependencies of logical relations.We perform a proof theoretical analysis for cut elimination, with new tools, and we explore some further important properties, which are not available in more traditional settings and which we can collectively regard as 'modularity'. Despite the complexities of the proof theoretical investigation, the system obtained is very simple. This paper contributes the following new results:
1Wedefine a propositional logical system, called NEL (Non-commutative exponential linear logic), which extends MELL (multiplicative exponential linear logic [8]) by a non-commutative, self-dual logical operator called seq. This system, which was first imagined in [10], is conservative over MELL augmented by the mix and nullary mix rules [1,6]. System NEL can be immediately understood by anybody acquainted with the sequent calculus, and is aimed at the same range of applications as MELL. In nearly all computer science languages, sequential composition plays a fundamental role,