  This paper proves a combinatorial rule expressing the product
$s_\tau(s_{\lambda/\mu} \circ p_r)$ of a Schur function and the plethysm of a
skew Schur function with a power sum symmetric function as an integral linear
combination of Schur functions. This generalizes the SXP rule for the plethysm
$s_\lambda \circ p_r$. Each step in the proof uses either an explicit bijection
or a sign-reversing involution. The proof is inspired by an earlier proof of
the SXP rule due to Remmel and Shimozono, A simple proof of the
Littlewood--Richardson rule and applications, Discrete Mathematics 193 (1998)
257--266. The connections with two later combinatorial rules for special cases
of this plethysm are discussed. Two open problems are raised. The paper is
intended to be readable by non-experts.
