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The question of when the double branched cover of a knot KÌýcan be constructedÌýby surgery on a knot in S^3 arises when oneÌýconsiders whether K can be obtained by rational tangleÌýreplacement on the unknot. I will explain how one can show that theÌýdouble branched cover of an alternatingÌýknot or linkÌýcan arise by non-integer surgery on a knot in S^3 if and only if it has an alternating diagram whichÌýcan be obtained from an almost-alternating diagram of the unknot by an appropriate tangle replacement. As a special case, this shows that the alternating knotsÌýwith unknotting number one are precisely those with an unknotting crossing in an alternating diagram.ÌýÌý