%{
#include <stdio.h>
#define YYERROR_VERBOSE
void yyerror(char *);
%}
%verbose
%token IF THEN ELSE EXPR ILLEGAL_CHAR
%%
                /* Dies ist die Lösung aus Kap. 5.3, Folie 26. */
statement:        unbalanced_st
                | balanced_st
                ;
unbalanced_st:    IF EXPR THEN statement
                | IF EXPR THEN balanced_st ELSE unbalanced_st
                ;
balanced_st:      IF EXPR THEN balanced_st ELSE balanced_st
                | other
                ;
other:            EXPR
                ;
%%
void yyerror(char *msg) {
    printf("\n%s\n", msg);
}
int main() {
    return yyparse();
}