code: func (b *Builder) Assign(replica, partition int, node *BuilderNode) {
  	b.ring.ReplicaToPartitionToNode[replica][partition] = lowring.Node(node.index)
  	b.ring.ReplicaToPartitionToWait[replica][partition] = 0
  }
docs: // Assign will override the current builder's assignment and set a specific
  // replica of a partition to a specific node. This is mostly just useful for
  // testing, as future calls to Rebalance may move this assignment.