  We use Young's raising operators to derive a Pieri rule for the ring
generated by the indeterminates $h_{r,s}$ given in Macdonald's 9th Variation of
the Schur functions. Under an appropriate specialisation of $h_{r,s}$, we
derive the Pieri rule for the ring $\La(a)$ of double symmetric functions,
which has a basis consisting of the double Schur functions. Together with a
suitable interpretation of the Jacobi--Trudi identity, our Pieri rule allows us
to obtain a new proof of a rule to calculate the Littlewood--Richardson
polynomials, which gives a multiplication rule for the double Schur functions.
