Provides an incrementing uint256 id acquired by the `Counter#next` getter. Use this for issuing ERC721 ids or keeping track of request ids, anything you want, really. * Include with `using Counter for Counter.Counter;`, Does not allow an Id of 0, which is popularly used to signify a null state in solidity. Does not protect from overflows, but if you have 2^256 ids, you have other problems. (But actually, it's generally impossible to increment a counter this many times, energy wise so it's not something you have to worry about.).
function next(Counter index) internal returns (uint256)