Correct an other bug

single/treewalk.cpp

@@ -55,11 +55,12 @@ void walk(int m,int k)
   
   auto it = generator_iter<CHILDREN>(temp);
   it.move_next();
-  ind_t pos=1;
+  ind_t pos=0;
   if(k==0){
     it.move_next();
     ++pos;
   }
+  --stack_pointer;
   while (it.move_next()){
     stack_pointer[0] = stack_pointer[1];
     remove_generator(**stack_pointer, temp, it.get_gen(),pos++);