Let Wa

$=$

$$Dom

$($

$\backslash$

phi a

$)$

$.$

$$Prove that there exists a recursive function h

$($

a

$)$

$$such that for every a: Wh

$($

a

$)$

$=$

$\{$

$0$

$,$

$$a

$,$

$2$

a

$,$

$.$

$\}$