Patrizio Frosini Claudia Landi Size Functions and Formal Series. 327-349 2001 12 Appl. Algebra Eng. Commun. Comput. 4 http://link.springer.de/link/service/journals/00200/bibs/1012004/10120327.htm db/journals/aaecc/aaecc12.html#FrosiniL01