  The pion mass difference generates a pronounced cusp in K --> 3 pi decays. As
has recently been pointed out by Cabibbo and Isidori, an accurate measurement
of the cusp may allow one to pin down the S-wave pi pi scattering lengths to
high precision. Here, we present and illustrate an effective field theory
framework that allows one to determine the structure of this cusp in a
straightforward manner. The strictures imposed by analyticity and unitarity are
respected automatically.
