@ ProgramNode (location: (1,0)-(3,20))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(3,20))
    └── body: (length: 2)
        ├── @ DefNode (location: (1,0)-(1,18))
        │   ├── name: :foo
        │   ├── name_loc: (1,4)-(1,7) = "foo"
        │   ├── receiver: ∅
        │   ├── parameters:
        │   │   @ ParametersNode (location: (1,8)-(1,12))
        │   │   ├── requireds: (length: 2)
        │   │   │   ├── @ RequiredParameterNode (location: (1,8)-(1,9))
        │   │   │   │   └── name: :_
        │   │   │   └── @ RequiredParameterNode (location: (1,11)-(1,12))
        │   │   │       └── name: :_
        │   │   ├── optionals: (length: 0)
        │   │   ├── rest: ∅
        │   │   ├── posts: (length: 0)
        │   │   ├── keywords: (length: 0)
        │   │   ├── keyword_rest: ∅
        │   │   └── block: ∅
        │   ├── body: ∅
        │   ├── locals: [:_]
        │   ├── locals_body_index: 1
        │   ├── def_keyword_loc: (1,0)-(1,3) = "def"
        │   ├── operator_loc: ∅
        │   ├── lparen_loc: (1,7)-(1,8) = "("
        │   ├── rparen_loc: (1,12)-(1,13) = ")"
        │   ├── equal_loc: ∅
        │   └── end_keyword_loc: (1,15)-(1,18) = "end"
        └── @ DefNode (location: (3,0)-(3,20))
            ├── name: :foo
            ├── name_loc: (3,4)-(3,7) = "foo"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (3,8)-(3,14))
            │   ├── requireds: (length: 2)
            │   │   ├── @ RequiredParameterNode (location: (3,8)-(3,10))
            │   │   │   └── name: :_a
            │   │   └── @ RequiredParameterNode (location: (3,12)-(3,14))
            │   │       └── name: :_a
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:_a]
            ├── locals_body_index: 1
            ├── def_keyword_loc: (3,0)-(3,3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (3,7)-(3,8) = "("
            ├── rparen_loc: (3,14)-(3,15) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (3,17)-(3,20) = "end"
