A species of structures is a rule which
The functions should further satisfy the following functorial properties:
An element is called an -structure on (or a structure of species on ). The function is called the transport of -structures along .
References
Bergeron, F.; Labelle, G.; and Leroux, P. Combinatorial Species and Tree-Like Structures.
Cambridge, England: Cambridge University Press, p. 5, 1998.