Add check for whether voter has right to vote

docs/solidity-by-example.rst

@@ -152,6 +152,7 @@ of votes.
         /// to proposal `proposals[proposal].name`.
         function vote(uint proposal) public {
             Voter storage sender = voters[msg.sender];
+            require(sender.weight != 0, "Has no right to vote");
             require(!sender.voted, "Already voted.");
             sender.voted = true;
             sender.vote = proposal;