@ ProgramNode (location: (1,0)-(1,14))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,14))
    └── body: (length: 1)
        └── @ CallOperatorWriteNode (location: (1,0)-(1,14))
            ├── receiver:
            │   @ CallNode (location: (1,0)-(1,3))
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── message_loc: (1,0)-(1,3) = "foo"
            │   ├── opening_loc: ∅
            │   ├── arguments: ∅
            │   ├── closing_loc: ∅
            │   ├── block: ∅
            │   ├── flags: variable_call
            │   └── name: "foo"
            ├── call_operator_loc: ∅
            ├── message_loc: (1,3)-(1,9) = "[0, 1]"
            ├── opening_loc: (1,3)-(1,4) = "["
            ├── arguments:
            │   @ ArgumentsNode (location: (1,4)-(1,8))
            │   └── arguments: (length: 2)
            │       ├── @ IntegerNode (location: (1,4)-(1,5))
            │       │   └── flags: decimal
            │       └── @ IntegerNode (location: (1,7)-(1,8))
            │           └── flags: decimal
            ├── closing_loc: (1,8)-(1,9) = "]"
            ├── flags: ∅
            ├── read_name: "[]"
            ├── write_name: "[]="
            ├── operator: :+
            ├── operator_loc: (1,10)-(1,12) = "+="
            └── value:
                @ IntegerNode (location: (1,13)-(1,14))
                └── flags: decimal