added beginning on 4-trees

slides/cours_19.md

@@ -365,7 +365,7 @@ Son équivalent tri-dimensionnel est l'octree (chaque noeud a 8 enfants ou aucun
 
 ```{.mermaid format=pdf width=400 loc=figs/}
 graph TD;
-    id0((0))-->id1((0));
+    id0((  ))-->id1((0));
     id0-->id2((  ));
     id0-->id3((  ));
     id0-->id4((  ));
@@ -381,8 +381,14 @@ graph TD;
     id4-->id14((0))
     id4-->id15((1))
     id4-->id16((  ))
-    style id8 fill:#fff,stroke:#fff
-    style id13 fill:#fff,stroke:#fff
+    id5-->id17((0))
+    id5-->id18((1))
+    id5-->id19((1))
+    id5-->id20((0))
+    id16-->id21((1))
+    id16-->id22((1))
+    id16-->id23((1))
+    id16-->id24((0))
 ```
 
 ::::

slides/figs/board_blacked_parts.svg

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