fluksio.arrived(): a node can tell which inputs the wave behind it wrote, so a value written again unchanged is seen

docs/code/nodes.md

@@ -92,6 +92,27 @@ Pass this callback to your training framework. It publishes through the same
 declared ports as `yield`. Outside a node, `emit` does nothing, so ordinary
 tests can still call your code.
 
+### `fluksio.arrived`
+
+Every input arrives as an argument, whether it was just written or only
+sits in state from before. `arrived()` names the input ports that the message
+which started this run wrote:
+
+```python
+import fluksio
+
+
+def process(auto, manual):
+    if "manual" in (fluksio.arrived() or ()):
+        return {"command": manual}
+    return {"command": auto}
+```
+
+A value written again unchanged counts as arrived. That is how a node tells
+somebody repeating a command apart from a value that has not moved. When the
+engine cannot say, such as when a flow starts, when a paused flow resumes, or
+outside a node, it returns `None`.
+
 ## Bytes: artifacts
 
 Save files or bytes with `save_artifact`, then return the reference through an