code: function pullBatch (renderer, pushable) {
    return next(renderer, 0, val => typeof val === "string" && pushable.push(val));
  }
docs: Given a batch size, request values from the source sequence and push them
  onto the provided "pushable" (a Node stream or an array).  Once the number
  of retrieved values reaches the specified batch size, return control back
  to the caller.
  
  @param      {Renderer}     renderer   The Renderer from which to pull next-vals.
  @param      {Array|Stream}            pushable   Destination for all segments.
  
  @return     {boolean|Promise}         Indicates whether there are more values to
  be retrieved, or if this the last batch.