[ ] -> 0
[ 1 ] -> 1
[ 1, 2 ] -> 2
[ 1, 2, 3 ] -> 3
[ 1 ] -> 1
[ 1, 2 ] -> 2
[ 1, 2, 3 ] -> 3
[ 1, 2, 3 ] -> 3
[ 1, 2, 3, 4 ] -> 4
[ 1, 2, 3, 4, 5 ] -> 5