Sequoia: A Playground for Logicians - (System Description).

Sequent calculus is a pervasive technique for studying logics and their properties due to the regularity of rules, proofs, and meta-property proofs across logics. However, even simple proofs can be large, and writing them by hand is often messy. Moreover, the combinatorial nature of the calculus makes it easy for humans to make mistakes or miss cases. Sequoia aims to alleviate these problems. Sequoia is a web-based application for specifying sequent calculi and performing basic reasoning about them. The goal is to be a user-friendly program, where logicians can specify and “play” with their calculi. For that purpose, we provide an intuitive interface where inference rules can be input in Open image in new window and are immediately rendered with the corresponding symbols. Users can then build proof trees in a streamlined and minimal-effort way, in whichever calculus they defined. In addition to that, we provide checks for some of the most important meta-theoretical properties, such as weakening admissibility and identity expansion, given that they proceed by the usual structural induction. In this sense, the logician is only left with the tricky and most interesting cases of each analysis.