added parent link, made tree mutable
added parent link, made tree mutable

dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas,dkandalov/katas