The purpose of the map is to help us find minimum SOP expressions,
where we defined minimum as being minimum number of product
terms (implicants), and among those with the same number of implicants,
the ones with the fewest number of literals. However, the only
product terms that we need consider are prime implicants. Why? Say we
found an implicant that was not a prime implicant. Then, it must be contained
in some larger implicant, a prime implicant. But that larger implicant
(say four 1’s rather than two) has fewer literals. That alone makes a