Cees,

> - BlockContext>>ensure: uses #valueUninterruptably to evaluate the
> block. It seems that the debugger cannot descent inside such 
> blocks.

Actually, this is legacy code for VMs which don't have the proper unwind
support (for another workaround of that kind see
#ifProperUnwindSupportedElseSignalAboutToReturn ;) As the comment says:

	<primitive: 123>
	"The above indicates new EH primitives supported. 
	In this case is identical to #value."

Therefore, you can safely remove any sends of #valueUninterruptably and
replace them by #value (which the debugger can step through).

Cheers,
  - Andreas