def first_filtered_bt_line(backtrace)
      ((fbt = backtrace.filtered).empty? ? backtrace : fbt).first.to_s
    end
def first_filtered_bt_line(backtrace)
      ((fbt = backtrace.filtered).empty? ? backtrace : fbt).first.to_s
    end
if the filtered backtrace is empty, just use the backtrace itself (this
 should only occur if the result is an error from a line in assert's
 non-test code).