library MathLib {

    function multiply(uint a, uint b) public view returns (uint, address) {
        return (a * b, address(this));
    }
}
contract Example {

    using MathLib for uint;
    address public owner = address(this);
    address public retAddr;

    function multiplyExample(uint _a, uint _b) public view returns (uint, address) {
        uint a;
        (a, retAddr) = _a.multiply(_b);
        return (a, retAddr);
    }
}