Take strings from an iterator and pass them to a writer function.
| Parameters | state | An iterator of str and Deferred. str instances will be passed to write. Deferred instances will be waited on before resuming iteration of state. |
| write | A callable which will be invoked with each str produced by iterating state. |
| result | A Deferred which will be called back when state has been completely flattened into write or which will be errbacked if an exception in a generator passed to state or an errback from a Deferred from state occurs. |
| Returns | None |