Prolog2

% 1
fst([H|_],H).

snd([_, S|_],S).

% 2
rev(Lst1,Lst2) :- revHelper(Lst1,Lst2,[]).

revHelper(Lst1,[],Lst1) :- !.
revHelper(Lst1,[H|T],Lst3) :-
    revHelper(Lst1,T,[H|Lst3]).

% 3
sym(Lst) :- rev(Lst,Lst).

% 4
odd(N) :- 0 =\= N mod 2.

prime(2).
prime(N) :-
    N>1,
    odd(N),
    checkPrime(3,N).

checkPrime(A,N) :-
    AA is A*A, AA>N;
    0 =\= N mod A,
    A2 is A+2,
    checkPrime(A2,N).

unprime(A) :- \+ prime(A).

primeList([],[]).
primeList([H|T],[PH|PT]) :-
    prime(H),
    H = PH,
    primeList(T,PT).
primeList([H|T],PLst) :-
    unprime(H),
    primeList(T,PLst),!.

% 5
pref([],_).
pref([H|T1],[H|T2]) :-
    pref(T1,T2).

sublst(Lst1,Lst2) :- pref(Lst1,Lst2), !.
sublst(Lst1,[_|T]) :- sublst(Lst1,T).

% 6
merge(Lst3,[],Lst3).
merge([],Lst3,Lst3).
merge([H|T1],[H|T2],[H,H|T3]) :- merge(T1,T2,T3), !.
merge([H|T1],Lst2,[H|T3]) :-
    Lst2 = [H2|_],
    H<H2,
    merge(T1,Lst2,T3).
merge(Lst1,[H|T2],[H|T3]) :-
    Lst1 = [H1|_],
    H1>H,
    merge(Lst1,T2,T3).

% 7
numlst(A,B) :- var(A), var(B), A = 1, B = [1].
numlst(N,Lst) :-
    var(Lst),
    numlstHelper(N,RLst),
    reverse(Lst,RLst).
numlst(N,Lst) :- reverse(Lst,RLst), numlstHelper(N,RLst).

numlstHelper(N,RLst) :- var(RLst), N<10, RLst = [N|[]].
numlstHelper(N,RLst) :-
    var(RLst),
    N10 is N mod 10,
    RLst = [N10|T],
    NN is N div 10,
    numlstHelper(NN,T).
numlstHelper(N,[H|[]]) :- N<10, N is H.
numlstHelper(N,[H|T]) :-
    H is N mod 10,
    NN is N div 10,
    numlstHelper(NN,T).