Frederic Bouvier:

Frederic Bouvier:

I just discovered this : state() is not valid when _level==0,
because it is doing:
{ return _state_stack[_state_stack.size() - 1]; }
and is returning a wrong index fetch ( 0 - 1 ) == -1

Moreover, st is not used when _level==0 so the line can be moved
to a safer place.