# Recursive combinatorial species

The combinatorial species of rooted trees can be defined recursively via $A = X\cdot E(A)$, where $X$ denotes the singleton species and $E$ the set species. How can I make the following code work? The last command yields an error.

```
sage: X = species.SingletonSpecies()
sage: E = species.SetSpecies()
sage: A = CombinatorialSpecies()
sage: A.define(X*E(A))
sage: A.generating_series()
```