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