  Using the example of selected decays driven by R-parity-violating
supersymmetric operators, we demonstrate how strong final-state interactions
can be controlled quantitatively with high precision, thus allowing for a more
accurate extraction of effective parameters from data. In our examples we focus
on the lepton-flavor-violating decays tau --> mu pi+ pi-. In R-parity violation
these can arise due to the product of two couplings. We find bounds that are an
order of magnitude stronger than previous ones.
