@ 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: (3...4) = "("
            ├── arguments:
            │   @ ArgumentsNode (location: (4...8))
            │   └── arguments: (length: 1)
            │       └── @ BlockArgumentNode (location: (4...8))
            │           ├── expression:
            │           │   @ CallNode (location: (5...8))
            │           │   ├── receiver: ∅
            │           │   ├── call_operator_loc: ∅
            │           │   ├── message_loc: (5...8) = "bar"
            │           │   ├── opening_loc: ∅
            │           │   ├── arguments: ∅
            │           │   ├── closing_loc: ∅
            │           │   ├── block: ∅
            │           │   ├── flags: variable_call
            │           │   └── name: "bar"
            │           └── operator_loc: (4...5) = "&"
            ├── closing_loc: (8...9) = ")"
            ├── block: ∅
            ├── flags: ∅
            └── name: "fun"
