match([_], []).
match([X|Xs], Ys) :-
    group_pairs_by_key(Y, [X-Xs]), append(Y, Y1, Ys), match(Xs, Y1).

game([], G, G).
game([X1-X2|Xs], Ys, G) :-
    append(Y1, [Y|Y2], Ys), \+ memberchk(Y, Y2),
    \+ memberchk(X1-_, Y), \+ memberchk(_-X1, Y),
    \+ memberchk(X2-_, Y), \+ memberchk(_-X2, Y),
    append(Y1, [[X1-X2|Y]|Y2], Z), game(Xs, Z, G).

round_robin(N, G) :- N mod 2 =:= 0,
    numlist(1, N, X), match(X, Y), N1 is N - 1,
    length(G0, N1), maplist(ord_empty, G0), game(Y, G0, G).

:-  writeln('4 teams'), forall(round_robin(4, G), writeln(G)),
    writeln('6 teams'), forall(round_robin(6, G), writeln(G)).