@InCollection{Skura:1996,
author
=
"T.Skura",
title
=
"Refutations and proofs in S4",
pages
=
"45--51",
year
=
"1996",
booktitle
=
"Proof Theory of Modal Logics ",
editor
=
"ed. H. Wansing",
publisher
=
"Kluwer Akademic Publishers",
address
=
"Dordrecht",
}