function issue(uint amount) public onlyOwner {

    //_totalSupply + amount must be greater than the _totalSupply value
    require(_totalSupply + amount > _totalSupply);

    //balance of owner + the amount must me greater than the owner's balance
    require(balances[owner] + amount > balances[owner]);

    balances[owner] += amount; // balances[owner]= amount+balances[amount]
    _totalSupply += amount; //_totalSupply = _totalSupply+amount
    emit Issue(amount); // when new token issues it will be emitted

function redeem(uint amount) public onlyOwner {

//to perform redeem successfully _totalSupplyis greater than or equal to the amount
    require(_totalSupply >= amount);

    // also require owner's balance greater than or equal to the amount
    require(balances[owner] >= amount);

    //_totalSupply = _totalSupply - amount;
    _totalSupply -= amount;

    //balances[owner] = balance[owner]-amount;
    balances[owner] -= amount;
   emit Redeem(amount);

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