Safek in Eruvei Techumin — Eruvin 35b
תלמוד בבלי מסכת עירובין דף לה/ב
אמר רבי דוסתאי בר ינאי משום רבי מאיר שמעתי שמקדרין בהרים
Since our 2000 amos limit on Techum Shabbos is a rabbinic stricture (according to Torah law the techum extends to 12 mil, or 24,000 amos), the law is that in measuring the techum we "bore into the mountains" — i.e., we measure the distance as it would be were the land in the area completely flat; we do not take the distance that the slope of a mountain adds into account.
A Mohel measured the distance to the city in which he had a Bris Milah to perform on Shabbos. Without "boring into the mountains," the distance was slightly more than an eruv techumin would allow. Teshuvos Achiezer () was asked whether the Mohel could rely on the assumption that the more lenient manner of measuring the distance would render the case permissible.
Achiezer initially suggests that since our measure of techum is a rabbinic decree, we may employ the principle that Safek d'Rabbanan l'Kullah — in a case of doubt that concerns a rabbinic decree we incline towards leniency. He then rejects this suggestion, as the principle is that any case of doubt in which the matter may be clarified (ikkah l'brurei), we do not incline towards leniency — and here, it is possible to measure the distance more accurately.
Achiezer ultimately rules leniently for several reasons. Among them is his rejection of the above rejection: From Tosafos to 5b above d.h. V'Safek it is clear that where the clarification of the matter in doubt is very difficult — in the case in question, it was winter and very snowy — it is not necessary to take extraordinary measures to clarify the matter and hence on may rely on the principle of Safek d'Rabbanan l'Kullah.