AppStorageExample.sol

// AppStorage.sol
struct AppStorage {
  uint256 secondVar;
  uint256 firstVar;
  uint256 lastVar;
  ...
}

// StakingFacet.sol
import "./AppStorage.sol"

contract Staking {
  AppStorage internal s;

  function myFacetFunction() external {
    s.lastVar = s.firstVar + s.secondVar;
  }
}