Model-independent description of $B\to D\pi \mathrm{\ell }\nu$ decays

In this contribution we present a novel, model-independent description of semileptonic $B\to D\pi \mathrm{\ell }\nu$ decays. In addition, we discuss recent developments in the understanding of coupled-channel $D\pi$-$D\eta$-$D_{s}K$ S-wave scattering and, for the first time, apply them to semileptonic decays. We not only obtain model-independent predictions for kinematic distributions in $B\to D\pi \mathrm{\ell }\nu$ decays, but also rule out the hypothesis that the gap between the inclusive $B\to X\mathrm{\ell }\nu$ branching fraction and the sum over exclusive channels is made up predominantly by $B\to D^{(\ast )}\eta \mathrm{\ell }\nu$ decays.