"creates a difference-list" should be "creates an open list ending in Tail if Tail is an unbound variable" (and something that is not a proper or open list otherwise, unless if Tail = [], then the result is a proper list)

attempt(File,Codes,X) :-
    set_prolog_flag(answer_write_options,[max_depth(0)]),
    open(File,read,Stream,[type(text),encoding(utf8)]),
    read_stream_to_codes(Stream,Codes,X),
    close(Stream).

?- attempt('small.txt',Codes,X).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10|$VAR(X)].

?- attempt('small.txt',Codes,X),X=[].
Codes = [73,116,39,115,32,115,109,97,108,108,33,10],
X = [].

?- attempt('small.txt',Codes,a).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10|a].

?- attempt('small.txt',Codes,[]).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10].