1 2 3 4 5 6 7 8 9 10
'a' to 'c' ok 'a' until 'c' ok 'a' to 'b' ok 'a' until 'b' ok 'a' to 'a' ok 'a' until 'a' ok 'b' to 'a' ok 'b' until 'a' ok 'c' to 'a' ok 'c' until 'a' ok