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<p>idea of looking at the syntactic features of sequents as special, non-logical operators is not old, and was forced by innovations in proof theory: when the structural operators are as simple as in Getzen's original sequent calculus there is little need to analyse them, but proof calculi of <a href="page.php?w=deep_inference">deep inference</a> such as <b>display logic</b> (introduced by <a href="page.php?w=Nuel_Belnap">Nuel Belnap</a> in 1982) support structural operators as complex as the logical connectives, and demand sophisticated treatment.</p><p>
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