Refactor TagFirstLast in terms of CountDown
Refactor TagFirstLast in terms of CountDown

The implementation is a lot simpler to follow and requires fewer
compositions.

ddpruitt/morelinq,morelinq/MoreLINQ,fsateler/MoreLINQ,morelinq/MoreLINQ,ddpruitt/morelinq,fsateler/MoreLINQ