A simple grammar.

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =14a(5) =42a(6) =138a(7) =466a(8) =1643a(9) =5919a(10) =21773a(11) =81279a(12) =307483a(13) =1175352a(14) =4534161a(15) =17626999a(16) =68992703a(17) =271641249a(18) =1075144364a(19) =4275274867a(20) =17071822275a(21) =68428152475a(22) =275217386092