お題はこちら http://qiita.com/items/4364285801d1c9f370a1

Haskellでパタンマッチングで書くことができたので、Prologに翻訳してみました（Haskellで投稿したものからちょっとだけ手を加えてあります、番兵のところ）。

テストの実行には https://github.com/hiratara/offline-DOUKAKU-skeletons のtest.prolog を利用してください。

answer.prolog

sitdown3([0'+, 0'-, 0'-|CS], P, [0'+, P, 0'-|CS]).
sitdown3([0'-, 0'-, 0'-|CS], P, [0'-, P, 0'-|CS]).
sitdown3([C|CS], P, [C|RS] ) :- sitdown3(CS, P, RS).

sitdown2([0'+, 0'-|CS], P, [0'+, P|CS]).
sitdown2([0'-, 0'-|CS], P, [P, 0'-|CS]).
sitdown2([C|CS], P, [C|RS] ) :- sitdown2(CS, P, RS).

sitdown1([0'-|CS], P, [P|CS] ).
sitdown1([C|CS], P, [C|RS] ) :- sitdown1(CS, P, RS).

sitdown(CS, P, RS) :- sitdown3(CS, P, RS), !.
sitdown(CS, P, RS) :- sitdown2(CS, P, RS), !.
sitdown(CS, P, RS) :- sitdown1(CS, P, RS), !.

leave([P|CS], P, [0'-|CS]).
leave([C|CS], P, [C|RS] ) :- leave(CS, P, RS).

traffic([], CS, CS).
traffic([P|PS], CS, RS) :- between(0'A, 0'Z, P), sitdown(CS, P, NS), traffic(PS, NS, RS).
traffic([P|PS], CS, RS) :- between(0'a, 0'z, P), to_upper(P, Q), leave(CS, Q, NS), traffic(PS, NS, RS).

sentinel(XS, H, T, [H|YS]) :- sentinel(XS, T, YS).
sentinel([], T, [T]).
sentinel([X|XS], T, [X|YS]) :- sentinel(XS, T, YS).

split(D, [D|XS], [], XS).
split(D, [X|XS], [X|YS], ZS) :- D \= X, split(D, XS, YS, ZS).

get_source(Input, Count, Source) :- split(0':, Input, NS, Source), number_chars(Count, NS).

replicate(0, _, []).
replicate(N, C, [C|CS]) :- N1 is N - 1, replicate(N1, C, CS).

solve(Input, Actual) :-
  get_source(Input, Count, Source),
  replicate(Count, 0'-, CS0),
  sentinel(CS0, 0'+, 0'-, CS1),
  traffic(Source, CS1, CS2),
  sentinel(Actual, 0'+, 0'-, CS2),
  format("~s~n", [Actual]).