An implicant is a rectangle of 1, 2, 4, 8, . . . 1’s or X’s (containing
no 0’s).
A prime implicant is an implicant not included in any one larger rectangle.
Thus, from the point of view of finding prime implicants, X’s
(don’t cares) are treated as 1’s.
An essential prime implicant is a prime implicant that covers at least
one 1 not covered by any other prime implicant (as always). Don’t
cares (X’s) do not make a prime implicant essential.