The Book of Mormon contains many examples of editorial previews which would be difficult to include if Joseph Smith had dictated it. One example is found Alma 21:17:
And it came to pass that the Lord began to bless them, insomuch that they brought many to the knowledge of the truth; yea, they did convince many of their sins, and of the traditions of their fathers, which were not correct. (emphasis added)
This wording is then used five chapters later in Alma 26:24
For they said unto us: Do ye suppose that ye can bring the Lamanites to the knowledge of the truth? Do ye suppose that ye can convince the Lamanites of the incorrectness of the traditions of their fathers, as stiffnecked a people as they are; whose hearts delight in the shedding of blood; whose days have been spent in the grossest iniquity; whose ways have been the ways of a transgressor from the beginning? Now my brethren, ye remember that this was their language. (emphasis added)
In this example and many others, Joseph Smith seems to recite content that has not even been written yet.
How did Joseph Smith remember the wording he used several chapters earlier? How did he manage to give a preview of future content then fulfill it later in the book?