contract MixFormulaInterface {
    function calcNewSkinAppearance(uint128 x, uint128 y) public pure returns (uint128);

    // create random appearance
    function randomSkinAppearance(uint256 externalNum) public view returns (uint128);

    // bleach
    function bleachAppearance(uint128 appearance, uint128 attributes) public pure returns (uint128);
}
function randomSkinAppearance(uint256 externalNum) public view returns (uint128);

// create random appearance