A simple grammar.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =11a(5) =29a(6) =88a(7) =271a(8) =876a(9) =2880a(10) =9699a(11) =33106a(12) =114575a(13) =400547a(14) =1413333a(15) =5025026a(16) =17987892a(17) =64769123a(18) =234438337a(19) =852519108a(20) =3113091384a(21) =11410682109a(22) =41967420293a(23) =154832147286a(24) =572853387928