@ ProgramNode (location: (0...11))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...11))
    └── body: (length: 1)
        └── @ DefNode (location: (0...11))
            ├── name: :f
            ├── name_loc: (4...5) = "f"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (6...7))
            │   ├── requireds: (length: 1)
            │   │   └── @ RequiredParameterNode (location: (6...7))
            │   │       └── name: :a
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:a]
            ├── def_keyword_loc: (0...3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: ∅
            ├── rparen_loc: ∅
            ├── equal_loc: ∅
            └── end_keyword_loc: (8...11) = "end"
