On Negative Tagging for Leastfixed - Point Formulae

Proof systems with sequents of the form U ` for proving validity of a propositional modal-calculus formula over a set U of states in a given model usually handle xed-point formulae through unfolding, thus allowing such formulae to reappear in a proof. Tagging is a technique originated by Winskel for annotating xed-point formulae with information about the proof states at which these are unfolded. This information is used later in the proof to avoid unnecessary unfolding, without having to investigate the history of the proof. Depending on whether tags are used for acceptance or for rejection of a branch in the proof tree, we refer to \positive" or \negative" tagging, respectively. In their simplest form, tags consist of the sets U at which xed-point formulae are unfolded. In this paper, we generalise results of earlier work by Andersen, Stirling and Winskel which, in the case of least xed-point formulae, are applicable to singleton U sets only.