Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number of elements in a mapping, issuing ERC721 ids, or counting request ids * Include with `using Counters for Counters.Counter;` Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the SafeMath overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never directly accessed.
function current(Counter counter) internal view returns (uint256)
function decrement(Counter counter) internal
function increment(Counter counter) internal