English
Called to ask the generator to produce buffers containing data for processing.

The returned value is the __ConstBufferSequence__ representing unconsumed data.

The function will ensure that `ec.failed()` is `false` if there was no error or set to the appropriate error code if there was one.

If no unconsumed data is available, this operation shall make progress to eventually reach completion.

The result of invoking `prepare` after completion or encountered error(s) is defined by the generator implementation. It can not be assumed to be meaningful or safe to do so, in general.

The capacity of the buffer returned is defined by the generator implementation.

[*Note:] Any buffers obtained by previous calls to `prepare` are invalidated.