-// node that a compaction builds, or is nil. It goes only to the usage file,
-// never to the endpoint.
+// node that a compaction builds, or is nil. Turn names the request of a turn:
+// first, next, or warmup. Node and Turn go only to the usage file, never to the
+// endpoint. A call with StopAtOutput ends at the first output item, after the
+// endpoint has read the whole input, and gives no output and no usage.