contract StandardToken is ERC20, BasicToken {

 
    mapping (address => mapping (address => uint256)) internal 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) public returns (bool) {

        require(_to != address(0));

        require(_value <= balances[_from]);

        require(_value <= allowed[_from][msg.sender]);

        balances[_from] = balances[_from].sub(_value);

        balances[_to] = balances[_to].add(_value);

        allowed[_from][msg.sender] = allowed[_from][msg.sender].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.
     * 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) {

        return allowed[_owner][_spender];

    }

 
}
function allowance(address _owner, address _spender) public view returns (uint256) {

    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 specifying the amount of tokens still available for the spender.
 */