Ошибка в работе программы

simbol_2N("",_).
 simbol_2N(Str,_):-frontchar(Str,S1,Ost),frontchar(Ost,S2,_),
                   S1=S2,write("V stroke est dva podrjad idushih odinakovuh simvola").
 simbol_2N(Str,_):-frontchar(Str,S1,Ost),frontchar(Ost,S2,_),S1<>S2,simbol_2N(Ost,_).
 simbol_2N(Str,_):- write("V stroke net dvuh podrjad idushih odinakovuh simvolov").

 process(4):-makewindow(5,7,7,"Proverka stroki",4,10,20,50),
             shiftwindow(5),
             write("Vvedite stroky:"),nl,
             readln(S),nl,
             simbol_2N(S,_),nl,
             write("Press any key"),readchar(_),
             removewindow.

 simbol_2N(Str):-
    frontchar(Str,S,Ost),frontchar(Ost,S,_),
    write("V stroke est dva podrjad idushih odinakovuh simvola"),!;
    simbol_2N(Ost),!.
 simbol_2N(Str):- write("V stroke net dvuh podrjad idushih odinakovuh simvolov").

 simbol_2N(St):-frontchar(St,S,Ost),frontchar(Ost,S,_),
        write("V stroke est dva podrjad idushih odinakovuh simvola"),!;
        simbol_2N(Ost),!.
 simbol_2N(St):-write("V stroke net dvuh podrjad idushih odinakovuh simvolov").

write("Vvedite stroky:"),nl,readln(Stroka),nl,simbol_2N(Stroka).

PREDICATES
p(char,char,string)
%
CLAUSES
%
p(A,A,_):- !.
p(_,A,B):- frontChar(B,C,D), p(A,C,D).

Goal: p(1,0,"a;ldskfj")
No
Goal: p(1,0,"a;ldskffj")
Yes

p(A,A,_):- !, write("bla-bla-bla ",A).
p(_,A,B):- frontChar(B,C,D), !, p(A,C,D). 
p(_,_,_):- write("figushki").

DOMAINS
c=char cl=c* s=string
%
PREDICATES
p(s,c,c,cl)
%
CLAUSES
%
p(A,B,B,[B|C]):- !, p(A,0,B,C).
p(A,_,B,C):- frontChar(A,D,E), !, p(E,B,D,C).
p(_,_,_,[]).

Goal: p("zemlya imet formu chemodana",1,0,Out)
Out=[]
1 Solution
Goal: p("zemlya imeet formu chemodana",1,0,Out)
Out=['e']
1 Solution
Goal: p("zemllya imeet formu chemmodana!!!",1,0,Out)
Out=['l','e','m','!','!']
1 Solution