Minimal working example (Re: set_buffer_mode)

 >{ inherit Stdio.Buffer;
 >  private final Thread.Condition _cond;
 >  private final Thread.Mutex _mux;
 >
 >  final void create(Thread.Mutex mux,Thread.Condition cond)
 >  { ::create(); _cond=cond; _mux=mux;
 >  }

I've noticed this recently, what is the idea behind calling
object-local variables _something? To mark them as private?