@ ProgramNode (location: (1,0)-(2,1))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(2,1))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(2,1))
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── message_loc: (1,0)-(1,3) = "fun"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (1,4)-(2,1))
            │   ├── arguments: (length: 1)
            │   │   └── @ ParenthesesNode (location: (1,4)-(2,1))
            │   │       ├── body:
            │   │       │   @ StatementsNode (location: (1,5)-(1,6))
            │   │       │   └── body: (length: 1)
            │   │       │       └── @ IntegerNode (location: (1,5)-(1,6))
            │   │       │           └── flags: decimal
            │   │       ├── opening_loc: (1,4)-(1,5) = "("
            │   │       └── closing_loc: (2,0)-(2,1) = ")"
            │   └── flags: ∅
            ├── closing_loc: ∅
            ├── block: ∅
            ├── flags: ∅
            └── name: :fun
