

"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].