  The Pieri rule expresses the product of a Schur function and a single row
Schur function in terms of Schur functions. We extend the classical Pieri rule
by expressing the product of a skew Schur function and a single row Schur
function in terms of skew Schur functions. Like the classical rule, our rule
involves simple additions of boxes to the original skew shape. Our proof is
purely combinatorial and extends the combinatorial proof of the classical case.
