@ ProgramNode (location: (0...9))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...9))
    └── body: (length: 1)
        └── @ CallNode (location: (0...9))
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── message_loc: (0...3) = "fun"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (4...6))
            │   └── arguments: (length: 1)
            │       └── @ ParenthesesNode (location: (4...6))
            │           ├── body: ∅
            │           ├── opening_loc: (4...5) = "("
            │           └── closing_loc: (5...6) = ")"
            ├── closing_loc: ∅
            ├── block:
            │   @ BlockNode (location: (7...9))
            │   ├── locals: []
            │   ├── parameters: ∅
            │   ├── body: ∅
            │   ├── opening_loc: (7...8) = "{"
            │   └── closing_loc: (8...9) = "}"
            ├── flags: ∅
            └── name: "fun"
