@ ProgramNode (location: (0...23))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...23))
    └── body: (length: 1)
        └── @ DefNode (location: (0...23))
            ├── name: :a
            ├── name_loc: (4...5) = "a"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (6...9))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest:
            │   │   @ ForwardingParameterNode (location: (6...9))
            │   └── block: ∅
            ├── body:
            │   @ StatementsNode (location: (12...18))
            │   └── body: (length: 1)
            │       └── @ CallNode (location: (12...18))
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── message_loc: (12...13) = "b"
            │           ├── opening_loc: (13...14) = "("
            │           ├── arguments:
            │           │   @ ArgumentsNode (location: (14...17))
            │           │   └── arguments: (length: 1)
            │           │       └── @ ForwardingArgumentsNode (location: (14...17))
            │           ├── closing_loc: (17...18) = ")"
            │           ├── block: ∅
            │           ├── flags: 
            │           └── name: "b"
            ├── locals: [:"..."]
            ├── def_keyword_loc: (0...3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (5...6) = "("
            ├── rparen_loc: (9...10) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (20...23) = "end"