DiamondStorageLib.sol

//SPDX-License-Identifier: MIT
pragma solidity =0.8.9;

library MyLib {
  function appStorage() 
    internal 
    pure 
    returns (AppStorage storage ds) 
  {    
    assembly {
      ds.slot := 0
    }
  }

  function someFunction() internal {
    AppStorage storage s = appStorage();
    ... do stuff
  }
}