contract StandardToken is ERC20, BasicToken {
        mapping(address => mapping(address => uint256)) allowed;
    
        /**
         * @dev Transfer tokens from one address to another
         * @param _from address The address which you want to send tokens from
         * @param _to address The address which you want to transfer to
         * @param _value uint256 the amount of tokens to be transferred
         */
        function _transferFrom(address _from, address _to, uint256 _value) internal returns (bool) {
            require(_to != address(0));
    
            uint256 _allowance = allowed[_from][msg.sender];
    
            // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
            // require (_value <= _allowance);
    
            balances[_from] = balances[_from].sub(_value);
            balances[_to] = balances[_to].add(_value);
            allowed[_from][msg.sender] = _allowance.sub(_value);
           emit Transfer(_from, _to, _value);
            return true;
        }
    
        /**
         * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
         *
         * Beware that changing an allowance with this method brings the risk that someone may use both the old
         * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
         * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
         * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
         * @param _spender The address which will spend the funds.
         * @param _value The amount of tokens to be spent.
         */
        function approve(address _spender, uint256 _value) public returns (bool) {
            allowed[msg.sender][_spender] = _value;
            emit Approval(msg.sender, _spender, _value);
            return true;
        }
    
        /**
         * @dev Function to check the amount of tokens that an owner allowed to a spender.
         * @param _owner address The address which owns the funds.
         * @param _spender address The address which will spend the funds.
         * @return A uint256 specifying the amount of tokens still available for the spender.
         */
        function allowance(address _owner, address _spender) public view returns (uint256 remaining) {
            return allowed[_owner][_spender];
        }
    
        /**
         * approve should be called when allowed[_spender] == 0. To increment
         * allowed value is better to use this function to avoid 2 calls (and wait until
         * the first transaction is mined)
         * From MonolithDAO Token.sol
         */
        function increaseApproval(address _spender, uint _addedValue) public returns (bool success) {
            allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
           emit  Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
            return true;
        }
    
        function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool success) {
            uint oldValue = allowed[msg.sender][_spender];
            if (_subtractedValue > oldValue) {
                allowed[msg.sender][_spender] = 0;
            } else {
                allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
            }
            emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
            return true;
        }
    }
/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
function approve(address _spender, uint256 _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
}

/**
 * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
 *
 * Beware that changing an allowance with this method brings the risk that someone may use both the old
 * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
 * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
 * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
 * @param _spender The address which will spend the funds.
 * @param _value The amount of tokens to be spent.
 */