Two-bell analog of A028355.

Terms

a(0) =1a(1) =2a(2) =12a(3) =121a(4) =212a(5) =1212a(6) =12121a(7) =21212a(8) =121212a(9) =1212121a(10) =2121212a(11) =12121212a(12) =121212121a(13) =212121212a(14) =1212121212a(15) =12121212121a(16) =21212121212a(17) =121212121212