Prolog Help Various Rules

[DavideBar]

Hello all! I really need some help of yours

I've got all these rules:

Returns all the vowels on a list

extractvowels([], []).
extractvowels([H|T], R) :- consonant(H), extractvowels(T, R).
extractvowels([H|T],[H|R]) :- \+ consonant(H), extractvowels(T, R).

Flattens a list:

flatten([],[]).
flatten([A|L],[A|L1]) :- xatom(A), flatten(L,L1).
flatten([A|L],R) :- flatten(A,A1), flatten(L,L1), append(A1,L1,R).

xatom(A) :- atom(A).
xatom(A) :- number(A).

Returns true or false:

member(X, [X|_]).      
member(X, [_|Tail]) :- member(X, Tail).

How can I type

extractvowels([s,a,r,a],X).

And get as result this

X = [a].

?

How can I join the rule

flatten

with the

extractvalues

?

And how can I create a rule that is able to remove the duplicates ?

PS: I can't use any of the built-in predicates