init(e, [
[1, 2, 3],
[4, 5, 6],
[7, 8, 0]
]).

?- flatten(S, FlatS), init(e, S).

S = [[1, 2, 3], [4, 5, 6], [7, 8, 0]]
FlatS = [[[1, 2, 3], [4, 5, 6], [7, 8, 0]]] ;

No
?- init(e, S), flatten(S, FlatS).            

S = [[1, 2, 3], [4, 5, 6], [7, 8, 0]]
FlatS = [1, 2, 3, 4, 5, 6, 7, 8, 0] ;

No
?-