how to construct a rudimentary function $h$ used in the paper ''scales in $L(\mathbb{R})$''

Question

A rudimentary function is a function finitely generated by the following schemata:

1. $f(\langle x_{1} , ... , x_{k} \rangle ) = x_{i}$,
2. $f(\langle x_{1} , ... , x_{k} \rangle ) = x_{i}\setminus x_{j}$,
3. $f(\langle x_{1} , ... , x_{k} \rangle )= \{ x_{i},x_{j}\}$
4. $f(\langle x_{1} , ... , x_{k} \rangle ) =h(g_{1}( x_{1},...,x_{k} ),...g_{l}(x_{1},...,x_{k})) $
5. $f(\langle x_{1} , ... , x_{k} \rangle ) = \bigcup_{y \in x_{1}} g( y,x_{2},...,x_{k} )$

For a list of properties of rudimentary functions see for example page 234 of http://www.math.cmu.edu/~laiken/papers/FineStructure.pdf

In Steel's paper ''Scales in $L(\mathbb{R})$'' he claims that it is easy to construct a rudimentary function $h$ such that for every finite $F \subseteq ON$: \begin{gather} h''\{F\}\times \mathbb{R} = \{ \langle G_{0},G_{1},G_{2} \rangle \ | G_{i} \subseteq F \ \text{for} \ 0 \leq i \leq 2\} \end{gather}

Please, could anyone give me a hint on how to construct such $h$?