Antonio Bucciarelli Benjamin Leperchey Vincent Padovani Relative Definability and Models of Unary PCF. 75-89 2003 conf/tlca/2003 TLCA http://link.springer.de/link/service/series/0558/bibs/2701/27010075.htm db/conf/tlca/tlca2003.html#BucciarelliLP03