A simple grammar.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =11a(5) =29a(6) =91a(7) =286a(8) =955a(9) =3235a(10) =11240a(11) =39559a(12) =141217a(13) =509059a(14) =1852248a(15) =6790372a(16) =25062536a(17) =93043286a(18) =347223013a(19) =1301777476a(20) =4900835727a(21) =18519492431a(22) =70220471264a(23) =267080436295