xmas(N) :-
	tree(0, N, L, []),
	string_to_list(S, L),
	write(S).

tree(N, N) -->
	{ S is N * 2 + 4 },
	"o", repeat("-", S), "o".

tree(0, N0) -->
	{ maplist(plus(0), [0,2,4], [S1,S2,S3]) },
	{ maplist(plus(N0), [2,1,0], [P1,P2,P3]) },
	{ N1 is 1 },
	repeat(" ", P1), "/", repeat(" ", S1), "\\",
	"\n",
	repeat(" ", P2), "/", repeat(" ", S2), "\\",
	"\n",
	repeat(" ", P3), "/", repeat(" ", S3), "\\",
	"\n",
	tree(N1, N0),
	"\n",
	{ P4 is N0 + 2},
	repeat(" ", P4), "||".

tree(N, N0) -->
	{ N < N0 },
	{ S is N0 - N - 1 },
	{ S0 is N * 2 },
	{ N1 is N + 1 },
	{ maplist(plus(S0), [0,2,4], [S1,S2,S3]) },
	{ maplist(plus(S), [1,2,1], [P1,P2,P3]) },
	repeat(" ", P1), "o-/", repeat(" ", S1), "\\-o",
	"\n",
	repeat(" ", P2), "/", repeat(" ", S2), "\\",
	"\n",
	repeat(" ", P3), "/", repeat(" ", S3), "\\",
	"\n",
	tree(N1, N0),
	"\n",
	{ P4 is N0 + 2},
	repeat(" ", P4), "||".

repeat(_, 0) --> [].
repeat(C, N) -->
	{ N > 0 },
	C,
	{ N1 is N - 1 },
	repeat(C, N1).