Andrzej Skowron Jaroslaw Stepaniuk James F. Peters Roman W. Swiniarski Calculi of Approximation Spaces. 363-378 2006 72 Fundam. Inform. 1-3 http://iospress.metapress.com/content/4fax7ha0b5tw237a/ db/journals/fuin/fuin72.html#SkowronSPS06