A simple grammar.

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =32a(5) =117a(6) =460a(7) =1850a(8) =7680a(9) =32467a(10) =139631a(11) =608280a(12) =2679881a(13) =11916492a(14) =53417407a(15) =241118245a(16) =1095030281a(17) =4999847240a(18) =22938576846a(19) =105690791104a(20) =488860041249