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