  The COMPASS spectrometer at CERN has collected a large data set for
diffractive three-pion production of $46\times10^6$ exclusive events. Based on
previous conventional Partial-Wave Analyses (PWA), we performed a `freed-isobar
PWA' on the same data, removing model assumptions on the dynamic isobar
amplitudes for dominating waves. In this analysis, we encountered continuous
mathematical ambiguities, which we were able to identify and resolve. This
analysis gives an unprecedented insight in the interplay of $2\pi$ and $3\pi$
dynamics in the process. As an example we show results for a spin-exotic wave
$J^{PC}_{X^-}=1^{-+}$ wave.
