@ ProgramNode (location: (0...16))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...16))
    └── body: (length: 1)
        └── @ DefNode (location: (0...16))
            ├── name: :f
            ├── name_loc: (4...5) = "f"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (6...10))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 1)
            │   │   └── @ KeywordParameterNode (location: (6...10))
            │   │       ├── name: :foo
            │   │       ├── name_loc: (6...10) = "foo:"
            │   │       └── value: ∅
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:foo]
            ├── def_keyword_loc: (0...3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (5...6) = "("
            ├── rparen_loc: (10...11) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (13...16) = "end"
