code: func newBucket(tx *Tx) Bucket {
  	var b = Bucket{tx: tx, FillPercent: DefaultFillPercent}
  	if tx.writable {
  		b.buckets = make(map[string]*Bucket)
  		b.nodes = make(map[pgid]*node)
  	}
  	return b
  }
docs: // newBucket returns a new bucket associated with a transaction.