A simple grammar: power set of pairs of sequences.

Terms

a(0) =1a(1) =0a(2) =1a(3) =2a(4) =3a(5) =6a(6) =9a(7) =16a(8) =24a(9) =42a(10) =63a(11) =102a(12) =157a(13) =244a(14) =373a(15) =570a(16) =858a(17) =1290a(18) =1930a(19) =2858a(20) =4228a(21) =6208a(22) =9084a(23) =13216a(24) =19175a(25) =27666a(26) =39804a(27) =57020a(28) =81412a(29) =115820