@ ProgramNode (location: (1,0)-(1,12))
├── locals: [:foo]
└── statements:
    @ StatementsNode (location: (1,0)-(1,12))
    └── body: (length: 1)
        └── @ LocalVariableOperatorWriteNode (location: (1,0)-(1,12))
            ├── name_loc: (1,0)-(1,3) = "foo"
            ├── operator_loc: (1,4)-(1,6) = "+="
            ├── value:
            │   @ CallNode (location: (1,7)-(1,12))
            │   ├── flags: ∅
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── name: :m
            │   ├── message_loc: (1,7)-(1,8) = "m"
            │   ├── opening_loc: ∅
            │   ├── arguments:
            │   │   @ ArgumentsNode (location: (1,9)-(1,12))
            │   │   ├── flags: ∅
            │   │   └── arguments: (length: 1)
            │   │       └── @ LocalVariableReadNode (location: (1,9)-(1,12))
            │   │           ├── name: :foo
            │   │           └── depth: 0
            │   ├── closing_loc: ∅
            │   └── block: ∅
            ├── name: :foo
            ├── operator: :+
            └── depth: 0
