A Sink is a stream that is not readable. You must have a sink at the end of a pipeline for data to move towards.
You can only use one sink per pipeline.
pull(source, through, sink)
Drain the stream, calling
op on each
done when stream is finished.
If op returns
===false, abort the stream.
reduce stream into single value, then callback.
Read the stream into an array, then callback.
Drain the stream and then callback when done.
output the stream to