Замена заглавных букв на строчные в SWI-Prolog

% ?- maplist(to_lower, "Hello, World!", Sr), format('~s', [Sr]).
% hello, world!
% Sr = [104, 101, 108, 108, 111, 44, 32, 119, 111|...].

word(Word)-->
    letter(Letter),
    letters(RestLetters),
    {
        string_to_list(Word, [Letter | RestLetters])
    }.
letters([Letter | RestLetters])-->
    letter(Letter),!,
    letters(RestLetters).
letters([])-->[].
letter(Letter)-->
    [Letter],
    {
        code_type(Letter, alpha)
    }.
sentence([Word, End], Current, Result)-->
    word(Word),
    {Result is Current + 1},
    endOfSentence(End),!.
sentence([Word | RestWords], Current, Result)-->
    word(Word),
    (" "; "\n"),
    {Sum is Current + 1},
    sentence(RestWords, Sum, Result).
endOfSentence(End)-->
    [End],
    {
        member(End, ".?!")
    }.
text([Sentence | RestText], Current, Result)-->
    sentence(Sentence, 0, WordsNumber),
    (" "; "\n"),
    {Sum is Current + WordsNumber},
    text(RestText, Sum, Result),!.
text([], Result, Result)-->[].
 
 
getContent(Buffer, Text):-
    not(at_end_of_stream),!,
    get_code(Symbol),
    append(Buffer, [Symbol], SymbolList),
    getContent(SymbolList, Text).
 
getContent(Buffer, Buffer):-
    at_end_of_stream,!.
 
numberOfWords(FileName, Result, Rest) :-
    see(FileName),
    getContent([], Text),
    seen,
    phrase(text(_, 0, Result), Text, Rest).