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