  For a partition $\nu$, let $\lambda,\mu\subseteq \nu$ be two distinct
partitions such that $|\nu/\lambda|=|\nu/\mu|=1$. Butler conjectured that the
divided difference
$\operatorname{I}_{\lambda,\mu}[X;q,t]=(T_\lambda\widetilde{H}_\mu[X;q,t]-T_\mu\widetilde{H}_\lambda[X;q,t])/(T_\lambda-T_\mu)$
of modified Macdonald polynomials of two partitions $\lambda$ and $\mu$ is
Schur positive. By introducing a new LLT equivalence called column exchange
rule, we give a combinatorial formula for
$\operatorname{I}_{\lambda,\mu}[X;q,t]$, which is a positive monomial
expansion. We also prove Butler's conjecture for some special cases.
