The sequence [2, 4, 6] is valid. Test other sequences to discover what makes a sequence valid. When you think you know, write down your guess, reveal the rule, and see how it compares.
(You should try to deduce the truth using as few tests as possible; however, your main priority is getting the rule right.)

[ , , ]


0) [2, 4, 6] is VALID

Your guess is . . .