Eruvin 37: Retroactive Designation

Terminology/What's What: Bereirah. Can an eruv techumin kick in retroactively, as determined by whether it is used? This kind of planning and thinking ahead, with a verbal declaration that designates a change at another time may be uniquely human, certainly far-ranging. Plus: A complete reversal of the sages' opinions, apparently after the fact - a "Fokh." A fluidity in the text itself that is no longer in evidence.