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Description:
In recent years a two-scale expansion was established to study reactions of the type NN -> NN pi within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this Letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator. (c) 2007 Elsevier B.V. All rights reserved.