CoW_token1.sol

// WRITE
function transfer(address _to, uint256 _value) public returns (bool) {
  updateCopyOnWriteMap(msg.sender, _to);
  return super.transfer(_to, _value);
}

function updateCopyOnWriteMap(address _from, address _to) internal {
  if(!isDataInMapping(_from)){
    balancesAtTheStartOfVoting[_for].balance = balances[_for];
  }
  
  if(!isDataInMapping(_to)){
    balancesAtTheStartOfVoting[_to].balance = balances[_to];
  }
}

// READ
function getBalance(address _for) public vire returns(uint256){
   return balances[_for];
}

function getBalanceAtEventStart(address _for) public view returns(uint256){
  if (isDataInMapping(_for)) {  
    return balancesAtTheStartOfVoting[_for].balance;
  }
  
  return balances[_for];
}

function isDataInMapping(address _for) internal view returns(bool) {
  // TODO
}