Cristian Calude Sergiu Rudeanu Proving as a Computable Procedure. 43-52 2005 64 Fundam. Inform. 1-4 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=64&issue=1&spage=43 db/journals/fuin/fuin64.html#CaludeR05