The “a fortiori” principle is used to create a “how much more so” effect. Donald Parry explains:
A fortiori deals with “the peculiar sequence of two numbers, the second number being one unit higher than the first number (x/x+1),” such as the sequence one/two, three/four, or thousand/ten thousand.– Donald W. Parry, Poetic Parallelisms in the Book of Mormon: The Complete Text Reformatted (Provo, UT: Neal A. Maxwell Institute for Religious Scholarship, 2007), xxix.
One example from the Bible is found in Genesis 4:24
If Cain shall be avenged sevenfold,
truly Lamech seventy and sevenfold.
A fortiori is also found in the Book of Mormon where two numbers are given in succession to create a “how much more so” effect. One example is found in Alma 60:22
Yea, will ye sit in idleness while ye are surrounded with thousands of those, yea, and tens of thousands, who do also sit in idleness
Another example is found within a chiasm in Helaman 3:24-26:
A. insomuch that there were thousands
B. who did join themselves unto the church
C. and were baptized unto repentance.
D. And so great was the prosperity of the church, and so many the blessings which were poured out upon the people,
E. that even the high priests
E. and the teachers were themselves astonished beyond measure.
D. And it came to pass that the work of the Lord did prosper
C. unto the baptizing
B. and uniting to the church of God, many souls,
A. yea, even tens of thousands.
Why would Joseph Smith repeat numbers like this? How would he remember the a fortiori principle even within a chiasm?