@ ProgramNode (location: (0...3))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...3))
    └── body: (length: 1)
        └── @ CallNode (location: (0...3))
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── message_loc: (0...3) = "foo"
            ├── opening_loc: ∅
            ├── arguments: ∅
            ├── closing_loc: ∅
            ├── block: ∅
            ├── flags: variable_call
            └── name: "foo"
