@ ProgramNode (location: (0...22))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...22))
    └── body: (length: 1)
        └── @ SingletonClassNode (location: (0...22))
            ├── locals: []
            ├── class_keyword_loc: (0...5) = "class"
            ├── operator_loc: (6...8) = "<<"
            ├── expression:
            │   @ CallNode (location: (9...12))
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── message_loc: (9...12) = "foo"
            │   ├── opening_loc: ∅
            │   ├── arguments: ∅
            │   ├── closing_loc: ∅
            │   ├── block: ∅
            │   ├── flags: variable_call
            │   └── name: "foo"
            ├── body:
            │   @ StatementsNode (location: (14...17))
            │   └── body: (length: 1)
            │       └── @ NilNode (location: (14...17))
            └── end_keyword_loc: (19...22) = "end"
