corresponding unconstrained observables in the Stapp-Ypsilanti-Metropolis (SYM or “nuclear-bar”) parametrisation are $$k^{2\pm1-2J}\bar{\delta}_{J\pm1}\;\;\mbox{ and }
  \;\;k^{-(2J+1)}\bar{\epsilon}_J\;\;.$$ In the Blatt-Biedenharn parametrisation, the same rules apply for the eigenphases, but $k^{-2}\epsilon_J$ is the unconstrained variable for the mixing angle; see e.g. [@deSwart:1995ui]. These choices do not suffer from unitarity constraints (except for being real below the first inelasticity) and can be used directly.

[**Dependence on Parameter Input**]{} Let us first consider processes in which ${\mathcal{O}}(k)$ is a parameter-free prediction, i.e. its LECs are all known from some other process(es). To what extent does the procedure depend on that choice? In the example, the two-nucleon interactions were determined to match the Z-parametrisation of $\mathrm{NN}$-scattering (fit to pole position and residue of the scattering amplitude) [@Phillips:1999hh]. Fig. \[fig:EREvsZ\] shows that results with Bethe’s Effective-Range parametrisation have a markedly different rate of convergence, but the extracted slopes and ${\overline{\Lambda}_{\slashed{\pi}}}$ agree very well [@improve3body].

[**Accidental Zeroes and Infinities**]{} Some observables may show additional structures which should be avoided. For example, the [${}^{3}\mathrm{P}_{0}$]{} phase shift in $\mathrm{NN}$ scattering is zero at a lab energy of about $150\;{\ensuremath{\mathrm{MeV}}}$, so that the relative deviation of ${\mathcal{O}}=\delta_l$ in eq.  diverges. Likewise, ${\mathcal{O}}=k^{2l+1}\cot\delta_l$ diverges (approaches zero) at $\delta_l=0$ ($\pi/2$), e.g. in the 