@ ProgramNode (location: (0...8))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...8))
    └── body: (length: 1)
        └── @ CallOperatorWriteNode (location: (0...8))
            ├── receiver:
            │   @ CallNode (location: (0...1))
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── message_loc: (0...1) = "a"
            │   ├── opening_loc: ∅
            │   ├── arguments: ∅
            │   ├── closing_loc: ∅
            │   ├── block: ∅
            │   ├── flags: variable_call
            │   └── name: "a"
            ├── call_operator_loc: ∅
            ├── message_loc: (1...3) = "[]"
            ├── opening_loc: (1...2) = "["
            ├── arguments: ∅
            ├── closing_loc: (2...3) = "]"
            ├── flags: ∅
            ├── read_name: "[]"
            ├── write_name: "[]="
            ├── operator: :+
            ├── operator_loc: (4...6) = "+="
            └── value:
                @ CallNode (location: (7...8))
                ├── receiver: ∅
                ├── call_operator_loc: ∅
                ├── message_loc: (7...8) = "b"
                ├── opening_loc: ∅
                ├── arguments: ∅
                ├── closing_loc: ∅
                ├── block: ∅
                ├── flags: variable_call
                └── name: "b"
