@ ProgramNode (location: (0...19))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...19))
    └── body: (length: 1)
        └── @ DefNode (location: (0...19))
            ├── name: :a
            ├── name_loc: (9...10) = "a"
            ├── receiver:
            │   @ SelfNode (location: (4...8))
            ├── parameters:
            │   @ ParametersNode (location: (11...15))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 1)
            │   │   └── @ KeywordParameterNode (location: (11...15))
            │   │       ├── name: :b
            │   │       ├── name_loc: (11...13) = "b:"
            │   │       └── value:
            │   │           @ IntegerNode (location: (14...15))
            │   │           └── flags: decimal
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:b]
            ├── def_keyword_loc: (0...3) = "def"
            ├── operator_loc: (8...9) = "."
            ├── lparen_loc: ∅
            ├── rparen_loc: ∅
            ├── equal_loc: ∅
            └── end_keyword_loc: (16...19) = "end"
