rot13(Str, SR) :-
maplist(rot, Str, Str1),
string_to_list(SR, Str1).
rot(C, C1) :-
( member(C, "abcdefghijklmABCDEFGHIJKLM") -> C1 is C+13;
( member(C, "nopqrstuvwxyzNOPQRSTUVWXYZ") -> C1 is C-13; C1 = C)).