contract StandardToken is ERC20, BasicToken {

	mapping (address => mapping (address => uint256)) internal allowed;

	function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
		require(_to != address(0));
		require(_from != _to);
		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;
	}

	// https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
	function approve(address _spender, uint256 _value) public returns (bool) {
		allowed[msg.sender][_spender] = _value;
		emit Approval(msg.sender, _spender, _value);
		return true;
	}

    // return how many tokens _owner approve to _spender
	function allowance(address _owner, address _spender) public view returns (uint256) {
		return allowed[_owner][_spender];
	}

    // increase approval to _spender
	function increaseApproval(address _spender, uint _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;
	}

    // decrease approval to _spender
	function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool) {
		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;
	}
}
// refer: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
function allowance(address _owner, address _spender) public view returns (uint256) {
	return allowed[_owner][_spender];
}

// return how many tokens _owner approve to _spender