A simple grammar.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =16a(5) =49a(6) =153a(7) =493a(8) =1623a(9) =5460a(10) =18625a(11) =64373a(12) =224843a(13) =792546a(14) =2815443a(15) =10070115a(16) =36233440a(17) =131063713a(18) =476320360a(19) =1738401482a(20) =6368750019a(21) =23413000913a(22) =86342448134a(23) =319329672079