I don't think I understand what you're asking.  [Here's what a `SerialDisposable` is](http://msdn.microsoft.com/en-us/library/system.reactive.disposables.serialdisposable%28v=vs.103%29.aspx): Represents a disposable whose underlying disposable can be swapped for another disposable which causes the previous underlying disposable to be disposed.  On step 4 the first `disp2` is disposed.  Am I missing something?