gBASIC. `help` for prompt commands, `quit` to leave.
> unknown
unknown
> type(unknown)
unknown
> answer = unknown
> if answer then
...   print("yes")
... else
...   print("no")
... end if
runtime error at <prompt>:1:1: unknown cannot be used as a condition
> print("still here")
still here
> is_unknown(answer)
true
> quit
Your program was not saved; `recover` will bring it back next time.
