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Introduction
============

Nonlinearities in wave mechanics are the source of fascinating phenomena, from instabilities [@Skipetrov200prl], harmonic generation [@Saltiel2008prl; @gonella17jmps; @khajehtourian2019nonlinear], extreme energy transfer [@ZHANG2018jmps] and non-reciprocal transmission [@Lepri2011prl], to the formation of shocks [@CHOCKALINGAM2020103746], rogue waves [@Baronio2012prl] and *solitary waves* [@SILLING2016jmps]. The latter are amplitude-dependent waves which propagate with a constant velocity and fixed shape, owing to a balance between dispersion and nonlinearity in the system [@dauxois2006physics]. Initially studied in fluid mechanics [@BOYD2015417], quantum mechanics [@Kasamatsu2006pra] and optics [@Stegeman1999science; @kivshar2003optical], solitary waves are recently gaining increased attention from the solid mechanics community [@nadkarni2014dynamics; @deng2018metamaterials; @DENG2020jmps; @Katz2019; @mo2019cnoidal]. This interest was triggered by the quest for mechanical *metamaterials*artificial composites with properties and functionalities not found in 