In Kunen's book *Foundation of Mathematics* the definition of a well formed expression (**wfe**) of a lexicon for Polish notation $\langle W, \alpha \rangle$ ($W$ is a set and $\alpha:W\to\omega$ is a function) is given by 

* If $s\in W\_n$ and $\tau\_0,\dots\tau\_{n-1}$ are **wfe** then $s\tau\_0\dots\tau\_{n-1}$ is a **wfe**;

where $W\_n$ is the collection of all $n$-ary elements of this lexicon for Polish notation , i.e. $W\_n = \alpha^{-1}[\{n\}]$.

He latter mention that this definition is given by recursion, using a function $F:\omega\to\wp(W^{<\omega})$, where $F(n)$ is the set of all **wfe** of length $n$; I tried to write down the function, but I did not succeeded since it seems that one should have the set of all **wfe** of $\langle W, \alpha \rangle$ before defining the **wfe** of $\langle W, \alpha \rangle$. I would appreciate any hint.