This is a solidity library with two functions and a struct.

libraryF.sol

pragma solidity ^0.4.24;

library count{
    struct hold{
        uint a;
     
    }
    
    function subUint(hold storage s, uint b) public  returns(uint){
        
        require(s.a >= b); // Make sure it doesn't return a negative value.
        return s.a - b;
        
    }
    function addUint(uint a , uint b) public pure returns(uint){
        
        uint c = a + b;
        
        require(c >= a);   // Makre sure the right computation was made
        return c;
    }
}