EBNF grammar structure?

Question

Mameha adds two odd prime numbers together.

a) Construct an EBNF grammar for the sum that Mameha is going to obtain (assume there is no leading zero in the sum).

Answer 1

If the Goldbach Conjecture is correct, Mameha could obtain any even number greater than 4. The single-digit results are "6" and "8". Otherwise, the possible sums can be described as:


Of course, this must be expressed in whatever form of EBNF you are using, but that part should be easy.