    def next_tokens(self) -> Set[int]:
        """Returns the list of tokens that could come next.
        These are (a) all tokens extending the root state and, for
        non-root states, additionally all tokens extending the current
        state."""

        tokens = set()
        if self.state > 0:
            tokens.add(self.sequence[0])
        if not self.finished:
            tokens.add(self.sequence[self.state + 1])
        return tokens
Returns the list of tokens that could come next.
These are (a) all tokens extending the root state and, for
non-root states, additionally all tokens extending the current
state.
fairseq/token_generation_constraints.py