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(_value <= balances[_from]);

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

    require(_to != address(0));



    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.

   * 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)

  {

    return allowed[_owner][_spender];

  }



  /**

   * @dev Increase the amount of tokens that an owner allowed to a 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

   * @param _spender The address which will spend the funds.

   * @param _addedValue The amount of tokens to increase the allowance by.

   */

  function increaseApproval(

    address _spender,

    uint256 _addedValue

  )

    public

    returns (bool)

  {

    allowed[msg.sender][_spender] = (

      allowed[msg.sender][_spender].add(_addedValue));

    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);

    return true;

  }



  /**

   * @dev Decrease the amount of tokens that an owner allowed to a spender.

   * approve should be called when allowed[_spender] == 0. To decrement

   * allowed value is better to use this function to avoid 2 calls (and wait until

   * the first transaction is mined)

   * From MonolithDAO Token.sol

   * @param _spender The address which will spend the funds.

   * @param _subtractedValue The amount of tokens to decrease the allowance by.

   */

  function decreaseApproval(

    address _spender,

    uint256 _subtractedValue

  )

    public

    returns (bool)

  {

    uint256 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.
 
 * https://github.com/ethereum/EIPs/issues/20
 
 * Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 
 */
function decreaseApproval(

    address _spender,

    uint256 _subtractedValue

  )

    public

    returns (bool)

  {

    uint256 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;

  }
/**
 
 * @dev Decrease the amount of tokens that an owner allowed to a spender.
 
 * approve should be called when allowed[_spender] == 0. To decrement
 
 * allowed value is better to use this function to avoid 2 calls (and wait until
 
 * the first transaction is mined)
 
 * From MonolithDAO Token.sol
 
 * @param _spender The address which will spend the funds.
 
 * @param _subtractedValue The amount of tokens to decrease the allowance by.
 
 */