TY  - DATA
AU  - Levy, Azriel
TI  - A hierarchy of formulas in set theory 
T2  - Memoirs of the American Mathematical Society, 
SN  - 9781470400033 (online)
PY  - 1965///
CY  - Providence, R.I.
PB  - American Mathematical Society
UR  - http://www.ams.org/memo/0057
ER  -