  We discuss the recent Cabibbo's proposal to measure the pion-pion scattering
lengths combination a0-a2 from the cusp effect in the pi0-pi0 energy spectrum
at threshold for K+ to (pi0 pi0 pi+) and KL to (pi0 pi0 pi0). We estimate the
theoretical uncertainty of the a0-a2 determination at NLO in our approach and
obtain that it is not smaller than 5% for K+ to (pi0 pi0 pi+). One gets similar
theoretical uncertainties if the neutral KL to (pi0 pi0 pi0) decay data below
threshold are used instead. For this decay, there are very large theoretical
uncertainties above threshold due to cancellations and data above threshold
cannot be used to get the scattering lengths.
