Binary Relations (reflexivity)
A binary relation on S is reflexive provided that for every element in S, the pair of that element with itself is a pair in S.
R4 = { (a, a), (a, b), (b, b), (b, c), (c, c) } is reflexive.
R5 = { (a, a), (a, b), (b, b) } is not reflexive, because c ? S but (c, c) ?R5.