@ ProgramNode (location: (1,0)-(1,28))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,28))
    └── body: (length: 1)
        └── @ DefNode (location: (1,0)-(1,28))
            ├── name: :f
            ├── name_loc: (1,4)-(1,5) = "f"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (1,6)-(1,16))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 1)
            │   │   └── @ OptionalKeywordParameterNode (location: (1,6)-(1,12))
            │   │       ├── flags: ∅
            │   │       ├── name: :a
            │   │       ├── name_loc: (1,6)-(1,8) = "a:"
            │   │       └── value:
            │   │           @ NilNode (location: (1,9)-(1,12))
            │   ├── keyword_rest:
            │   │   @ KeywordRestParameterNode (location: (1,14)-(1,16))
            │   │   ├── flags: ∅
            │   │   ├── name: ∅
            │   │   ├── name_loc: ∅
            │   │   └── operator_loc: (1,14)-(1,16) = "**"
            │   └── block: ∅
            ├── body:
            │   @ StatementsNode (location: (1,19)-(1,24))
            │   └── body: (length: 1)
            │       └── @ CallNode (location: (1,19)-(1,24))
            │           ├── flags: ignore_visibility
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── name: :b
            │           ├── message_loc: (1,19)-(1,20) = "b"
            │           ├── opening_loc: (1,20)-(1,21) = "("
            │           ├── arguments:
            │           │   @ ArgumentsNode (location: (1,21)-(1,23))
            │           │   ├── flags: contains_keyword_splat
            │           │   └── arguments: (length: 1)
            │           │       └── @ KeywordHashNode (location: (1,21)-(1,23))
            │           │           ├── flags: ∅
            │           │           └── elements: (length: 1)
            │           │               └── @ AssocSplatNode (location: (1,21)-(1,23))
            │           │                   ├── value: ∅
            │           │                   └── operator_loc: (1,21)-(1,23) = "**"
            │           ├── closing_loc: (1,23)-(1,24) = ")"
            │           └── block: ∅
            ├── locals: [:a]
            ├── def_keyword_loc: (1,0)-(1,3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (1,5)-(1,6) = "("
            ├── rparen_loc: (1,16)-(1,17) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (1,25)-(1,28) = "end"