  A new approach to the analysis of three body decays is presented.
Model-independent results are obtained for the \swave $K\pi$ amplitude as a
function of $K\pi$ invariant mass. These are compared with results from
$\Km\pip$ elastic scattering, and the prediction of the Watson theorem, that
the phase behavour be the same below $K\eta^{\prime}$ threshold, is tested.
Contributions from $I=\half$ and $I={3\over 2}$ are not resolved in this study.
If $I=\half$ dominates, however, the Watson theorem does not describe these
data well.}
