Modify winner update check.

lesson3_violations/Borda/BordaMissingRule.spec

@@ -0,0 +1,18 @@
+methods {
+    function points(address) external returns uint256  envfree;
+    function vote(address,address,address) external;
+    function voted(address) external returns bool  envfree;
+    function winner() external returns address  envfree;
+}
+
+rule drawFavorsWinner(env e, address f, address s, address t) {
+    address w1 = winner(e);
+
+    uint256 w1Points = points(e, w1);
+
+    vote(e, f, s, t);
+
+    address w2 = winner(e);
+
+    assert w1 != w2 => points(e, f) > w1Points || points(e, s) > w1Points || points(e, t) > w1Points;
+}

lesson3_violations/Borda/BordaNewBug.sol

@@ -0,0 +1,48 @@
+pragma solidity ^0.8.0;
+import "./IBorda.sol";
+
+contract Borda is IBorda {
+
+    // The current winner
+    address public _winner;
+
+    // A map storing whether an address has already voted. Initialized to false.
+    mapping (address => bool)  _voted;
+
+    // Points each candidate has recieved, initialized to zero.
+    mapping (address => uint256) _points;
+
+    // current maximum points of all candidates.
+    uint256 public pointsOfWinner;
+
+
+    function vote(address f, address s, address t) public override {
+        require(!_voted[msg.sender], "this voter has already cast its vote");
+        require( f != s && f != t && s != t, "candidates are not different");
+        _voted[msg.sender] = true;
+        voteTo(f, 3);
+        voteTo(s, 2);
+        voteTo(t, 1);
+    }
+
+    function voteTo(address c, uint256 p) private {
+        //update points
+        _points[c] = _points[c] + p;
+        // update winner if needed
+        if (_points[c] >= _points[_winner]) { // Draw still changes winner.
+            _winner = c;
+        }
+    }
+
+    function winner() external view override returns (address) {
+        return _winner;
+    }
+
+    function points(address c) public view override returns (uint256) {
+        return _points[c];
+    }
+
+    function voted(address x) public view override returns(bool) {
+        return _voted[x];
+    }
+}