function to return vesting information for a caller.
     * Callers can then validate if their information has been properly stored,
     * instead of trusting the contract owner.
     */
    function check() external view returns (uint256, uint256, uint256, uint256) {
        return (
            _info[msg.sender].totalAmount, 
            _info[msg.sender].receivedAmount,
            _info[msg.sender].startTime, 
            _info[msg.sender].releaseTime
        );
    }