@TechReport{abel:tutch,
  author = 	 {Andreas Abel and Bor-Yuh Evan Chang and Frank Pfenning},
  title = 	 {Human-Readable Machine-Verifiable Proofs for
Teaching Constructive Logic},
  institution =  {Universit\`a degli Studi Siena, Dipartimento di Ingegneria dell'Informazione},
  year = 	 2001,
  key =		 {13/01},
  note =	 {In Proceedings of the Workshop on Proof Transformation and Presentation and Proof Complexities (PTP'01)},
  url =		 {http://www.tcs.informatik.uni-muenchen.de/\~{}abel/ptp01.pdf}
}