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 amout of tokens to be transfered
        */
        function transferFrom(address _from, address _to, uint256 _value) returns (bool) {
            var _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[_to] = balances[_to].add(_value);
            balances[_from] = balances[_from].sub(_value);
            allowed[_from][msg.sender] = _allowance.sub(_value);
            Transfer(_from, _to, _value);
            return true;
        }

        /**
        * @dev Aprove the passed address to spend the specified amount of tokens on behalf of msg.sender.
        * @param _spender The address which will spend the funds.
        * @param _value The amount of tokens to be spent.
        */
        function approve(address _spender, uint256 _value) returns (bool) {

            // To change the approve amount you first have to reduce the addresses`
            //  allowance to zero by calling `approve(_spender, 0)` if it is not
            //  already 0 to mitigate the race condition described here:
            //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
            require((_value == 0) || (allowed[msg.sender][_spender] == 0));
    
            allowed[msg.sender][_spender] = _value;
            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 specifing the amount of tokens still available for the spender.
        */
        function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
            return allowed[_owner][_spender];
        }

    }
/**
 * @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 allowance(address _owner, address _spender) constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
}

/**
 * @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 specifing the amount of tokens still available for the spender.
 */