A simple grammar.

Terms

a(0) =1a(1) =1a(2) =5a(3) =31a(4) =229a(5) =1832a(6) =15583a(7) =137791a(8) =1255202a(9) =11693697a(10) =110905169a(11) =1067181020a(12) =10392861567a(13) =102239342761