Stefan Hetzl Alexander Leitsch Proof Transformations and Structural Invariance. 201-230 2006 conf/birthday/2006mundici Algebraic and Proof-theoretic Aspects of Non-classical Logics http://dx.doi.org/10.1007/978-3-540-75939-3_13 db/conf/birthday/mundici2006.html#HetzlL06