  We reprove and extend a result of David Krebes (J. Knot Theory Ramif. 8
(1999), 321-352) giving an obstruction to embedding a tangle T into a link L.
Closing the tangle up in the two obvious ways gives rise to two links, the
numerator and denominator links n(T) and d(T). Applying a homological argument
to the 2-fold branched covers of these links, we show that the gcd of the
determinants of the numerator and denominator divides the determinant of L.
