  We generalize the problem of recovering a lost/erased symbol in a
Reed-Solomon code to the scenario in which some side information about the lost
symbol is known. The side information is represented as a set $S$ of linearly
independent combinations of the sub-symbols of the lost symbol. When $S =
\varnothing$, this reduces to the standard problem of repairing a single
codeword symbol. When $S$ is a set of sub-symbols of the erased one, this
becomes the repair problem with partially lost/erased symbol. We first
establish that the minimum repair bandwidth depends on $|S|$ and not the
content of $S$ and construct a lower bound on the repair bandwidth of a linear
repair scheme with side information $S$. We then consider the well-known
subspace-polynomial repair schemes and show that their repair bandwidths can be
optimized by choosing the right subspaces. Finally, we demonstrate several
parameter regimes where the optimal bandwidths can be achieved for full-length
Reed-Solomon codes.
