A simple grammar.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =6a(5) =16a(6) =49a(7) =156a(8) =511a(9) =1723a(10) =5942a(11) =20817a(12) =73938a(13) =265611a(14) =963225a(15) =3521667a(16) =12966696a(17) =48037862a(18) =178936340a(19) =669748197a(20) =2517688316a(21) =9501290664a(22) =35982633397a(23) =136708135198a(24) =520914346328