Basic addition parsing walkthrough

 2+20
^
--------------
state_: Begin
cache_: ""

2+20
^
--------------
state_: Integer
cache_: 2

2+20
 ^
--------------
   +
   |
   2
state_: Nary operator
cache_: ""

2+20
  ^
--------------
   +
   |
   2
state_: Integer
cache_: "2"

2+20
   ^
--------------
   +
   |
   2
state_: Integer
cache_: "20"

2+20
    ^
--------------
  +
 / \
2   20
state_: Done
cache_: ""