-// Creates a map from block locator to PullServers with one entry for
-// each under-replicated block.
+// ComputePullServers creates a map from block locator to PullServers
+// with one entry for each under-replicated block.
+//
+// This method ignores zero-replica blocks since there are no servers
+// to pull them from, so callers should feel free to omit them, but
+// this function will ignore them if they are provided.