Re: [Vm-dev] [OpenSmalltalk/opensmalltalk-vm] pushRcvr fails for context instances whose sender has been set to an Integer instance (Issue #654)

Ah, of course. I understand now. Setting to an integer has disastrous consequences for the vm. That must be prevented.

Possibly by coincidence Jaromir brought up the issue on squeak-dev, suggesting

```Context>>privSender: aContextOrNil

sender := aContextOrNil ifNotNil: [aContextOrNil asContext]```

which I think is good. I would validate by sending asContext in the relevant places. If necessary we can introduce something only understood by Context and not by BlockClosure, eg raiseErrorIfNotContext, which I suppose could be implemented in Object to raise an error.