@ ProgramNode (location: (1,0)-(1,13))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,13))
    └── body: (length: 1)
        └── @ UndefNode (location: (1,0)-(1,13))
            ├── names: (length: 1)
            │   └── @ SymbolNode (location: (1,6)-(1,13))
            │       ├── flags: forced_us_ascii_encoding
            │       ├── opening_loc: (1,6)-(1,9) = "%s("
            │       ├── value_loc: (1,9)-(1,12) = "foo"
            │       ├── closing_loc: (1,12)-(1,13) = ")"
            │       └── unescaped: "foo"
            └── keyword_loc: (1,0)-(1,5) = "undef"