Kees Doets Monadic Pi<sub>1</sub><sup>1</sup>-Theories of Pi<sub>1</sub><sup>1</sup>-Properties. 224-240 1989 30 Notre Dame Journal of Formal Logic 2 db/journals/ndjfl/ndjfl30.html#Doets89 http://projecteuclid.org/euclid.ndjfl/1093635080