  We consider processes which produce final state hadrons whose energy is much
greater than their mass. In this limit interactions involving collinear
fermions and gluons are constrained by a symmetry, and we give a general set of
rules for constructing leading and subleading invariant operators. Wilson
coefficients C(mu,P) are functions of a label operator P, and do not commute
with collinear fields. The symmetry is used to reproduce a two-loop result for
factorization in B -> D pi in a simple way.
