  The impulse-approximation expression used hitherto to define the pion's
valence-quark distribution function is flawed because it omits contributions
from the gluons which bind quarks into the pion. A corrected leading-order
expression produces the model-independent result that quarks dressed via the
rainbow-ladder truncation, or any practical analogue, carry all the pion's
light-front momentum at a characteristic hadronic scale. Corrections to the
leading contribution may be divided into two classes, responsible for shifting
dressed-quark momentum into glue and sea-quarks. Working with available
empirical information, we use an algebraic model to express the principal
impact of both classes of corrections. This enables a realistic comparison with
experiment that allows us to highlight the basic features of the pion's
measurable valence-quark distribution, $q^\pi(x)$; namely, at a characteristic
hadronic scale, $q^\pi(x) \sim (1-x)^2$ for $x\gtrsim 0.85$; and the
valence-quarks carry approximately two-thirds of the pion's light-front
momentum.
