option invoke debugger on fail

[erjoalgo]

I'm aware of rove:*debug-on-error*p, which invokes the debugger when a test raises a condition.

I have used stefil before, which supports also entering the debugger when a test fails. I find this convenient since I can immediately inspect expressions within the frame and this makes correcting things quicker.

Is this possible in prove, or if not would you consider adding it?

#10