A5: g in PrimRec by Def19;
A6: 2 <= 1 + 1 ;
A7: arity g = 1 + 2 by Def27;
A8: arity f = 1 by Def25;
f in PrimRec by Def19;
hence primrec f,g,2 in PrimRec by A6, A8, A7, A5, Th77; :: according to COMPUT_1:def 19 :: thesis: primrec f,g,2 is binary
thus arity (primrec f,g,2) = 2 by A6, A8, A7, Th61; :: according to COMPUT_1:def 26 :: thesis: verum