docs: def log(self, statement):
          '''Write statement to stderr with timing info in DEBUG mode.'''
          global DEBUG
          time_ms = 1000 * time.time()
          if DEBUG:
              total_time_ms = time_ms - self._start_time_ms_
              lap_time_ms = time_ms - self._last_time_ms_
              sys.stderr.write('%6.f (%6.f ms) %s\n' % (
                  total_time_ms, lap_time_ms, statement))
          self._last_time_ms_ = time_ms