docs: def maybeDeferred(f, *args, **kw):
      """
      Copied from twsited.internet.defer and add a check to detect fibers.
      """
      try:
          result = f(*args, **kw)
      except Exception:
          return fail(failure.Failure())
  
      if IFiber.providedBy(result):
          import traceback
          frames = traceback.extract_stack()
          msg = "%s returned a fiber instead of a deferred" % (f, )
          if len(frames) > 1:
              msg += "; called from %s" % (frames[-2], )
          raise RuntimeError(msg)
  
      if isinstance(result, Deferred):
          return result
      elif isinstance(result, failure.Failure):
          return fail(result)
      else:
          return succeed(result)