code: def flatten(source, task_limit=None):
      
      return base_combine.raw(
          source, task_limit=task_limit, switch=False, ordered=False)
docs: Given an asynchronous sequence of sequences, generate the elements
      of the sequences as soon as they're received.
  
      The sequences are awaited concurrently, although it's possible to limit
      the amount of running sequences using the `task_limit` argument.
  
      Errors raised in the source or an element sequence are propagated.