A simple grammar.

Terms

a(0) =1a(1) =1a(2) =4a(3) =19a(4) =107a(5) =647a(6) =4167a(7) =27847a(8) =191747a(9) =1349743a(10) =9671316a(11) =70297105a(12) =517079157a(13) =3841701488a(14) =28787546360a(15) =217317367487