'keys' over "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"
7 contains 8
9 contains 10
1 contains 2
5 contains 6
3 contains 4

'values' over "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"
key contains 6
key contains 10
key contains 2
key contains 4
key contains 8