  A new approach to the analysis of three body decays is presented.
Measurements of the S-wave K pi amplitude are made in independent ranges of
invariant mass from threshold up to the upper kinematic limit in D+ to K-pi+pi+
decays. These are compared with results obtained from a fit where the S-wave is
assumed to have kappa(800) and K*0(1430) resonances. Results are also compared
with measurements of K-pi+ elastic scattering. Contributions from I = 1/2 and I
= 3/2 are not resolved in this study. If I = 1/2 dominates, however, the Watson
theorem prediction, that the phase behavour below K-eta' threshold should match
that in elastic scattering, is not well supported by these data. Production of
K-pi+ from these D decays is also studied.
