Perl 6 - How to combine two negates charachter classes in a grammar

[holli]

Suppose I have

use v6;

grammar G {
    token not_a { <-[ a ]> }
    token not_b { <-[ b ]> }
}

How do i implement a token neither_a_nor_b as a composition of not_a and not_b?

[moritz]

Just like in Perl 5 you can use look-aheads:

token neither_a_nor_b {
   <?before <not_a>>
   <?before <not_b>>
   .
}

If you define your define your character classes positively, you can also combine them directly:

token a { <[ a ]> }
token b { <[ b ]> }
token neither_a_nor_b { <-a-b> }

(don't know if anybody implements that, though.