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