A post-publication acknowledgment: Connection Scheme B was inspired by
an additive source-wise spanner due to M. Elkin (unpublished). In his
construction, for any set of sources S, the distances between pairs in S×S are
preserved to within +2 stretch; the size of the spanner is O(n|S|1/2). Theorem 4.2
generalizes this construction in multiple ways. One consequence is that
distances between pairs in S×V can be preserved to within +O(log n)
with the same size spanner.