Use ML patterns to write a function union $($S$1,$ S$2)$ that returns the union of sets S$1$ and S$2.$ We implement sets as unordered lists of elements, without repetitions. Hint: use$/$call the function member from Q$1$ to check whether an element belongs to a set or not.

I used this code as my function member for Q$1$:

fun member$($e$,$L$)=$

if null L

then false

else if e$=$ hd$($L$)$

then true

else member$($e$,$tl$($L$))$;