An interpretation of species as exponential generating functions, which count labelled structures.
Extract the coefficients of an exponential generating function as
a list of Integers. Since
EGF is an instance of
Species, the idea is that
labelled can be applied directly to
an expression of the Species DSL. In particular,
labelled s !!
n is the number of labelled s-structures on an underlying set of
size n. For example:
> take 10 $ labelled octopi [0,1,3,14,90,744,7560,91440,1285200,20603520]
gives the number of labelled octopi on 0, 1, 2, 3, ... 9 elements.