soliditylibrary2.sol

pragma solidity ^0.4.24;

library C {
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

contract A {
    function a() constant returns (uint256) {
        uint256 x = 50;
        return C.add(50, x);
    }
}