  We show that Szczarba's twisting cochain for a twisted Cartesian product is
essentially the same as the one constructed by Shih. More precisely, Szczarba's
twisting cochain can be obtained via the basic perturbation lemma if one uses a
'reversed' version of the classical Eilenberg-MacLane homotopy for the
Eilenberg-Zilber contraction. Along the way we prove several new identities
involving these homotopies.
