@ ProgramNode (location: (0...40))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...40))
    └── body: (length: 2)
        ├── @ DefNode (location: (0...18))
        │   ├── name: :foo
        │   ├── name_loc: (4...7) = "foo"
        │   ├── receiver: ∅
        │   ├── parameters:
        │   │   @ ParametersNode (location: (8...12))
        │   │   ├── requireds: (length: 2)
        │   │   │   ├── @ RequiredParameterNode (location: (8...9))
        │   │   │   │   └── name: :_
        │   │   │   └── @ RequiredParameterNode (location: (11...12))
        │   │   │       └── name: :_
        │   │   ├── optionals: (length: 0)
        │   │   ├── rest: ∅
        │   │   ├── posts: (length: 0)
        │   │   ├── keywords: (length: 0)
        │   │   ├── keyword_rest: ∅
        │   │   └── block: ∅
        │   ├── body: ∅
        │   ├── locals: [:_]
        │   ├── def_keyword_loc: (0...3) = "def"
        │   ├── operator_loc: ∅
        │   ├── lparen_loc: (7...8) = "("
        │   ├── rparen_loc: (12...13) = ")"
        │   ├── equal_loc: ∅
        │   └── end_keyword_loc: (15...18) = "end"
        └── @ DefNode (location: (20...40))
            ├── name: :foo
            ├── name_loc: (24...27) = "foo"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (28...34))
            │   ├── requireds: (length: 2)
            │   │   ├── @ RequiredParameterNode (location: (28...30))
            │   │   │   └── name: :_a
            │   │   └── @ RequiredParameterNode (location: (32...34))
            │   │       └── name: :_a
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:_a]
            ├── def_keyword_loc: (20...23) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (27...28) = "("
            ├── rparen_loc: (34...35) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (37...40) = "end"
