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
        isUsable
        onlyActive(msg.sender)
        onlyActive(_from)
        onlyActive(_to)
        returns (bool)
    {
        require(0 < _value, "StandardToken.transferFrom: Zero value");
        require(_value <= balances[_from], "StandardToken.transferFrom: Insufficient fund");
        require(_value <= allowed[_from][msg.sender], "StandardToken.transferFrom: Insufficient allowance");

        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
        isUsable
        onlyActive(msg.sender)
        onlyActive(_spender)
        returns (bool)
    {
        require(0 < _value, "StandardToken.approve: Zero value");

        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
        isUsable
        onlyActive(msg.sender)
        onlyActive(_spender)
        returns (bool)
    {
        require(0 < _addedValue, "StandardToken.increaseApproval: Zero value");

        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
        isUsable
        onlyActive(msg.sender)
        onlyActive(_spender)
        returns (bool)
    {
        require(0 < _subtractedValue, "StandardToken.decreaseApproval: Zero value");

        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.
 * @dev https://github.com/ethereum/EIPs/issues/20
 */
function transferFrom(address _from, address _to, uint256 _value) public
        isUsable
        onlyActive(msg.sender)
        onlyActive(_from)
        onlyActive(_to)
        returns (bool)
    {
        require(0 < _value, "StandardToken.transferFrom: Zero value");
        require(_value <= balances[_from], "StandardToken.transferFrom: Insufficient fund");
        require(_value <= allowed[_from][msg.sender], "StandardToken.transferFrom: Insufficient allowance");

        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 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
 */