Merge branch 'master' of kompiler.org:python/graph_drawing

TODO

@@ -0,0 +1,11 @@
+ - The graph of "--ab + c" is also not valid:
+
+     -
+     │
+      +
+    ╭─┴╮
+    *  c
+   ╭┴╮
+   - b
+   │
+   a