A prime implicant is an implicant that (from the point of view of the
map) is not fully contained in any one other implicant. For example, it is
a rectangle of two 1’s that is not part of a single rectangle of four 1’s. On
Map 3.13, all of the prime implicants of F are circled. They are
ABCD, ABC, ABD, and CD. Note that the only minterm that is not
part of a larger group is m0 and that the other four implicants that are
groups of two 1’s are all part of the group of four.