@ 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))
                ├── arguments: (length: 1)
                │   └── @ CallNode (location: (1,7)-(1,21))
                │       ├── receiver: ∅
                │       ├── call_operator_loc: ∅
                │       ├── message_loc: (1,7)-(1,10) = "fun"
                │       ├── opening_loc: ∅
                │       ├── arguments:
                │       │   @ ArgumentsNode (location: (1,11)-(1,14))
                │       │   ├── arguments: (length: 1)
                │       │   │   └── @ CallNode (location: (1,11)-(1,14))
                │       │   │       ├── receiver: ∅
                │       │   │       ├── call_operator_loc: ∅
                │       │   │       ├── message_loc: (1,11)-(1,14) = "foo"
                │       │   │       ├── opening_loc: ∅
                │       │   │       ├── arguments: ∅
                │       │   │       ├── closing_loc: ∅
                │       │   │       ├── block: ∅
                │       │   │       ├── flags: variable_call
                │       │   │       └── name: :foo
                │       │   └── flags: ∅
                │       ├── 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"
                │       ├── flags: ∅
                │       └── name: :fun
                └── flags: ∅
