docs: function onChunk(count) {
        // If chunk read has no bytes than there is nothing left, so end a
        // collection.
        if (count === 0) return next(end)
  
        // Move a offset `position` with `count` towards the end unless
        // position was a `null` in which case we just keep it (`null` means
        // from a current position).
        position = position === null ? position : position + count
        // Read chunk is forwarded to a consumer that will return
        // a promise which is delivered once write is complete.
        // In a future we may switch to an approach similar to new
        // streams in node, where buffering watermarks are specified
        // via options and reads can buffer up up to that point in
        // parallel to write.
        when(next(buffer.slice(0, count), state), onDrain, onError)
      }