    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        require(spender != address(0), "Cannot increaseAllowance for 0x0 address");
        _allowed[msg.sender][spender] = _allowed[msg.sender][spender].add(addedValue);
        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
        return true;
    }