From @ronaldxs

Override of internal character class in gramma won't combine with other
character classes​:

grammar g{
  token alpha {<[2]>};
  token alpha1 {<[2]>}; # same as alpha but without internal car
class conflict
  token beta { <[q]> };
  token delta {<+alpha +beta>};
  token delta1 {<+alpha>};
  token delta2 {<+alpha1 +beta>}
}
say so "2" ~~ /<g​::delta1>/; # OK
say so "2" ~~ /<g​::delta2>/; # OK
say so "2" ~~ /<g​::delta>/; # think wrong - should be true
say so "a" ~~ /<g​::delta>/; # true probably wrong but maybe
understandable