  The pion mass difference generates a pronounced cusp in the pi0 pi0 invariant
mass distribution of K+ --> pi0 pi0 pi+ decays. As originally pointed out by
Cabibbo, an accurate measurement of the cusp may allow one to pin down the
S-wave pion-pion scattering lengths to high precision. We present the
non-relativistic effective field theory framework that permits to determine the
structure of this cusp in a straightforward manner, including the effects of
radiative corrections. Applications of the same formalism to other decay
channels, in particular eta and eta' decays, are also discussed.
