Relation Algebra, why is this PDL sentence equivalent

Dec 2018
42
2
Amsterdam
In the book modal logic for open minds by johan van benthem there is on page 161 a statement that the sentence $\langle (R\lor S)* \rangle \phi$ is equivalent to the sentence $\langle (R* ; S*)* \rangle \phi$
(* means iteration and ; means composition here)

So:
$\langle (R\lor S)* \rangle \phi \equiv \langle (R* ; S*)* \rangle \phi$

Why is this equivalent to each other?