@ ProgramNode (location: (0...21))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...21))
    └── body: (length: 1)
        └── @ ReturnNode (location: (0...21))
            ├── keyword_loc: (0...6) = "return"
            └── arguments:
                @ ArgumentsNode (location: (7...21))
                └── arguments: (length: 1)
                    └── @ CallNode (location: (7...21))
                        ├── receiver: ∅
                        ├── call_operator_loc: ∅
                        ├── message_loc: (7...10) = "fun"
                        ├── opening_loc: ∅
                        ├── arguments:
                        │   @ ArgumentsNode (location: (11...14))
                        │   └── arguments: (length: 1)
                        │       └── @ CallNode (location: (11...14))
                        │           ├── receiver: ∅
                        │           ├── call_operator_loc: ∅
                        │           ├── message_loc: (11...14) = "foo"
                        │           ├── opening_loc: ∅
                        │           ├── arguments: ∅
                        │           ├── closing_loc: ∅
                        │           ├── block: ∅
                        │           ├── flags: variable_call
                        │           └── name: "foo"
                        ├── closing_loc: ∅
                        ├── block:
                        │   @ BlockNode (location: (15...21))
                        │   ├── locals: []
                        │   ├── parameters: ∅
                        │   ├── body: ∅
                        │   ├── opening_loc: (15...17) = "do"
                        │   └── closing_loc: (18...21) = "end"
                        ├── flags: ∅
                        └── name: "fun"
