in reply to "Current position" of source filter input relative to original input

The solution in the end was trivially simple, but it was dg on #p5p who pointed out that just inspecting caller should give me the information, and so it does.

An alternative approach suggested by mst on #p5p was to first inject BEGIN { Querylet::set_code_start(__LINE__) } as the first filter result, and then, after that has been evaluated, to return the rest of the code. That would have been a viable hack, but far less elegant.