Kalvis Apsitis Rusins Freivalds Raimonds Simanovskis Juris Smotrovs Closedness Properties in EX-Identification of Recursive Functions. 46-60 http://link.springer.de/link/service/series/0558/bibs/1501/15010046.htm 1998 conf/alt/1998 ALT db/conf/alt/alt1998.html#ApsitisFSS98