A. Leitsch, S. Hetzl:
"Proof Transformations and Structural Invariance";
in: "LNAI 4460: Algebraic and Proof-theoretic Aspects", S. Aguzzoli et al. (ed.); Springer, LNAI 4460, 2007, 201 - 230.
http://www.springerlink.com/content/c387343458431241/fulltext.pdf