  An hallmark of present-day Dyson-Schwinger equation applications in hadron
physics is the existence of a systematic and symmetry preserving truncation
scheme. This enables the proof of exact results; e.g., the leptonic decay
constant of every pseudoscalar meson except the pion vanishes in the chiral
limit. Calculations using the scheme's leading-order truncation are reliable in
the vector and flavour nonsinglet pseudoscalar channels. In this rainbow-ladder
truncation, an impulse approximation provides the consistent current for all
six-point quark Schwinger functions. That is well illustrated via the anomalous
process pi0 -> gamma gamma. Using two-, three- and four-point Schwinger
functions calculated in rainbow-ladder truncation, the textbook value of the
width is obtained algebraically and independent of model details if, and only
if, the impulse approximation is used to describe the associated matrix
element.
