message: Construct output streams from any stream manager
diff: @@ -60,6 +60,14 @@ public:
  // nop
  }
  + output_stream(stream_slot in_slot, stream_slot out_slot,
  + stream_manager_ptr mgr)
  + : in_(in_slot),
  + out_(out_slot),
  + ptr_(std::move(mgr)) {
  + // nop
  + }
  +
  // -- properties -------------------------------------------------------------
  /// Returns the slot of the origin stream if `ptr()` is a stage or 0 if
  