Proof that the set of all universal CFGs is co-RE complete
In the proof we construct grammar $g(n)$ such that it has same alphabet as the Turing machine + symbols for states + separator "#", which is defined to be: 

$\Sigma_n = \{0, 1, \triangleright, B, \#, q_0, q_1 . . , q_n \}$

So essentially $\Sigma_n = \Sigma_{g(n)}$. $\Sigma_n$ is a superset of $\{0, 1, \triangleright, B\}$.