  We show that certain differences of products of $P$-partition generating
functions are positive in the basis of fundamental quasi-symmetric functions
L_\alpha. This result interpolates between recent Schur positivity and monomial
positivity results of the same flavor. We study the case of chains in detail,
introducing certain ``cell transfer'' operations on compositions and an
interesting related ``L-positivity'' poset. We introduce and study
quasi-symmetric functions called ``wave Schur functions'' and use them to
establish, in the case of chains, that the difference of products we study is
itself equal to a single generating function K_{P,\theta} for a labeled poset
(P,\theta). In the course of our investigations we establish some factorization
properties of the ring of quasisymmetric functions.
