// after multiple grows.
						useY = top & 1
						top = tophash(hash)
					} else {
						if hash&newbit != 0 {
							useY = 1
						}
					}
				}

				if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
					throw("bad evacuatedN")
				}

				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
				dst := &xy[useY]                 // evacuation destination