Animations: reach of a row – scene background, placed container content, (missing) (#109)

Animations.md

@@ -3,7 +3,13 @@ scene. It has **rows**, each aiming at one property of the root itself or of an
 inside it (a container's or scene's tree), and each row has **keys**: a frame number and a value.
 It is **played on an object** (`Play animation`), and its rows reach that object and what its
 template placed inside it, so every placed instance of a container plays the same animation on its
-own. A scene animation reaches the scene's objects by id while that scene runs.
+own. A row reaches **into containers placed inside the root**, however deep — a container's row
+names a path of template ids, which every instance resolves below itself. A scene animation reaches
+the scene's objects by id while that scene runs, the content of its placed containers included (the
+same ids whether play bakes that content or the edit mode resolves it), and it can key the
+**scene itself**: its own properties such as *Background*, but no position, since a scene is placed
+nowhere. When a container template loses the object a row aims at, the row reads *(missing)* like
+any other whose target is gone.
 
 Its two properties are **Frames per second** (default 10) and **Playback**: *Loop*, *Once (hold)*
 (ends on the last value and leaves it), *Once (reset)* (ends and puts back what every target held