Hello SeqFans, this is, I guess, the first such seq, lexicographically ordered. Seq starts: 2,1,4,3,6,5,8,9,10,13,12,11,16,15,14,21,18,19,22,23,20,25,24,29,28,27,26,31,30... (always take the smallest a(n) not yet present and not leading to a contradiction) A071065 has the same property. Best, E.