def validate_votes(self, validators_H):
        "set of validators may change between heights"
        assert self.sender
        if not self.round_lockset.num_eligible_votes == len(validators_H):
            raise InvalidProposalError('round_lockset num_eligible_votes mismatch')
        for v in self.round_lockset:
            if v.sender not in validators_H:
                raise InvalidProposalError('invalid signer')
set of validators may change between heights