anchor.diff:    Subscribes #[b callback] to ending off an observable.
   
   p.
  -  If observable already ended, #[b fn] will be called immediately (synchronously).
  +  If observable already ended, #[b callback] will be called immediately (synchronously).
   
   pre(title='example').
     var stream = Kefir.sequentially(1000, [1, 2]);
  
positive.code: 
  p.
    If called on a property, which has a current value,
    the #[b fn] function will be called immediately (synchronously)
    with that value as an argument.
  
  pre(title='example').
  
negative.code: 
  p.
    If called on a property, which has a current error,
    #[b callback] will be called immediately (synchronously)
    with that error as an argument.
  
  pre(title='example').
  