According to John Tvedtnes, an author may promise in the course of writing to return to a subject later to supply further details. Actually keeping such a promise can prove difficult. Even with modern writing aids, memory can betray a person into failing to tuck in the corners of plot or information. Mormon, the editor of much of the Book of Mormon as we have it, made these types of promises at least seven times. In each case, he or his son Moroni followed through perfectly.
The preaching mission of the sons of Mosiah was related in Alma 17-25, eighteen chapters after Mormon had said in Mosiah 28:9 and 19-20 that he would later tell about it. [John A. Tvedtnes, "Mormon's Editorial Promises," in Rediscovering the Book of Mormon, p. 29]