Jo Erskine Hannay Axiomatic Criteria for Quotients and Subobjects for Higher-Order Data Types. 903-917 2003 conf/icalp/2003 ICALP http://link.springer.de/link/service/series/0558/bibs/2719/27190903.htm db/conf/icalp/icalp2003.html#Hannay03