code: def _data_in_prefetch_buffers(self, offset):
          
          k = [i for i in self._prefetch_data.keys() if i <= offset]
          if len(k) == 0:
              return None
          index = max(k)
          buf_offset = offset - index
          if buf_offset >= len(self._prefetch_data[index]):
              # it's not here
              return None
          return index
docs: if a block of data is present in the prefetch buffers, at the given
          offset, return the offset of the relevant prefetch buffer.  otherwise,
          return None.  this guarantees nothing about the number of bytes
          collected in the prefetch buffer so far.