  We consider tree diagram contributions to neutral pion production in pp ->pp
pi0 using chiral perturbation theory in the relativistic and extremely
non-relativistic limits. In marked difference with the results from heavy
fermion formalism, the impulse and s-wave rescattering terms in the
relativistic limit have equal signs and therefore add constructively, giving
rise to a substantial contribution to the cross section. We argue that the
power series expansion of the nucleon propagator is on the border of its
convergence circle. Consequently, a finite order heavy fermion formalism does
not predict nucleon pole terms correctly and therefore can not be applied to
meson production.
