We have a graph of updates that must be look like this, (possibly some
of the branches are degenerate):
-
- O===B0====*U--------B1******A
- \ \
- \ \
- `--L----T0 `--T1
-
+
+ O====X=========BA
+ \
+ \
+ `--K----L
+
+ O====X==========****B***A
+ \
+ \
+ `--K----L
+
+ O====X==========****B
+ \ \
+ \ \
+ `--K----L `----T
+
+ O====X==========****B--B
+ \ \
+ \ \
+ `--K----L `-T
+
+
+
+ O====X==========****B***A
+ \
+ \
+ `--K----L
+
+ O====X=========A****B***A
+ \
+ \
+ `--K----L
+
+ O====X=========******R******A
+ \
+ \
+ `--K----L
+
+ O====X=============A
+ \ \
+ \ \
+ `--K----L `----T
+
There is zero or one B1T1 and zero or more B0LT0 (all distinct).
We write just B to mean B1, or the last B0.