Salvatore Caporaso Michele Zito Nicola Galesi Emanuele Covino Syntactic Characterization in LISP of the Polynominal Complexity Classes and Hierarchy. 61-73 1997 conf/ciac/1997 CIAC db/conf/ciac/ciac1997.html#CaporasoZGC97 http://dx.doi.org/10.1007/3-540-62592-5_61