code: def _notify_reader_writes(writeto):
      
      satisfied = []
      for var in writeto:
          if var.readable:
              for reader in var.readers:
                  reader.notify_read_ready()
                  if reader.satisfied:
                      satisfied.append(reader)
      return Closure.sort(satisfied)
docs: Notify reader closures about these writes and return a sorted
         list of thus-satisfied closures.