pick edge (a,b)  ->  cover gains a and b
pick edge (c,d)  ->  cover gains c and d
pick edge (e,f)  ->  cover gains e and f

approximate cover C = a b c d e f   |C| = 6   (matching size 3)
optimal cover C* = a c d e   |C*| = 4

ratio |C|/|C*| = 1.5000   (the theorem guarantees at most 2)
check |C*| >= matching size:  4 >= 3
check |C| = 2 * matching:     6 = 6
