  The classical hook length formula of enumerative combinatorics expresses the
number of standard Young tableaux of a given partition shape as a single
fraction. In recent years, two generalizations of this formula have emerged:
one by Pak and Postnikov, replacing the number by a (rational) generating
function, and one by Naruse, which generalizes the setting from a partition to
a skew partition. Both generalizations appear to lie significantly deeper, with
no simple proofs known. We combine them into a generating-function identity for
skew partitions, and prove it in a fairly elementary way using recursion,
determinants and simple combinatorics.
