prolog term without a functor
transform([a,b],X)

=..((a,b,c,d),X)

transform([A], A):-
  A=..[_].
transform([A,B], (A,B)):-
  B=..[_].
transform([A,B,C|Tail], L):-
  L=..[',',A,T],
  transform([B,C|Tail], T).

?- transform([a,b], X).
X = (a, b)

?- transform([a,b,c,d,e,f], X).
X = (a, b, c, d, e, f)

?- transform(L, (a,b,c,d,e,f,g))
L = [a, b, c, d, e, f, g]