@ ProgramNode (location: (1,0)-(1,18))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,18))
    └── body: (length: 1)
        └── @ IndexOperatorWriteNode (location: (1,0)-(1,18))
            ├── flags: ∅
            ├── receiver:
            │   @ CallNode (location: (1,0)-(1,3))
            │   ├── flags: variable_call, ignore_visibility
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── name: :foo
            │   ├── message_loc: (1,0)-(1,3) = "foo"
            │   ├── opening_loc: ∅
            │   ├── arguments: ∅
            │   ├── closing_loc: ∅
            │   └── block: ∅
            ├── call_operator_loc: ∅
            ├── opening_loc: (1,3)-(1,4) = "["
            ├── arguments:
            │   @ ArgumentsNode (location: (1,4)-(1,8))
            │   ├── flags: ∅
            │   └── 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) = "]"
            ├── block: ∅
            ├── operator: :+
            ├── operator_loc: (1,10)-(1,12) = "+="
            └── value:
                @ CallNode (location: (1,13)-(1,18))
                ├── flags: ignore_visibility
                ├── receiver: ∅
                ├── call_operator_loc: ∅
                ├── name: :m
                ├── message_loc: (1,13)-(1,14) = "m"
                ├── opening_loc: ∅
                ├── arguments:
                │   @ ArgumentsNode (location: (1,15)-(1,18))
                │   ├── flags: ∅
                │   └── arguments: (length: 1)
                │       └── @ CallNode (location: (1,15)-(1,18))
                │           ├── flags: variable_call, ignore_visibility
                │           ├── receiver: ∅
                │           ├── call_operator_loc: ∅
                │           ├── name: :foo
                │           ├── message_loc: (1,15)-(1,18) = "foo"
                │           ├── opening_loc: ∅
                │           ├── arguments: ∅
                │           ├── closing_loc: ∅
                │           └── block: ∅
                ├── closing_loc: ∅
                └── block: ∅
