code: async def filterindex(source, func):
      
      source = transform.enumerate.raw(source)
      async with streamcontext(source) as streamer:
          async for i, item in streamer:
              if func(i):
                  yield item
docs: Filter an asynchronous sequence using the index of the elements.
  
      The given function is synchronous, takes the index as an argument,
      and returns ``True`` if the corresponding should be forwarded,
      ``False`` otherwise.