2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/143144We show how to "concatenate" variational principles over different bases into one over a single base, thereby providing a unified Lagrangian treatment of interacting systems. As an example we study a Klein-Gordon field interacting with a mesically charged particle. We employ our method to give a novel group-theoretic derivation of the kinetic stress-energy-momentum tensor density corresponding to the particle.16 pages, 1 figureMathematical Physics70S05; 70G75text