Binary Relations (symmetry)
A binary relation R on S is symmetric provided that any pair that occurs in R also occurs “reversed” in R.
R6 = { (a, b), (b, a), (c, c) } is symmetric.
R7 = { (a, b), (b, b), (c, c) } is not symmetric, because (a, b) ? R7 but (b, a) ?R7.