@ ProgramNode (location: (0...14))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...14))
    └── body: (length: 1)
        └── @ LambdaNode (location: (0...14))
            ├── locals: [:arg]
            ├── operator_loc: (0...2) = "->"
            ├── opening_loc: (12...13) = "{"
            ├── closing_loc: (13...14) = "}"
            ├── parameters:
            │   @ BlockParametersNode (location: (3...11))
            │   ├── parameters:
            │   │   @ ParametersNode (location: (4...10))
            │   │   ├── requireds: (length: 0)
            │   │   ├── optionals: (length: 1)
            │   │   │   └── @ OptionalParameterNode (location: (4...10))
            │   │   │       ├── name: :arg
            │   │   │       ├── name_loc: (4...7) = "arg"
            │   │   │       ├── operator_loc: (7...8) = "="
            │   │   │       └── value:
            │   │   │           @ HashNode (location: (8...10))
            │   │   │           ├── opening_loc: (8...9) = "{"
            │   │   │           ├── elements: (length: 0)
            │   │   │           └── closing_loc: (9...10) = "}"
            │   │   ├── rest: ∅
            │   │   ├── posts: (length: 0)
            │   │   ├── keywords: (length: 0)
            │   │   ├── keyword_rest: ∅
            │   │   └── block: ∅
            │   ├── locals: (length: 0)
            │   ├── opening_loc: (3...4) = "("
            │   └── closing_loc: (10...11) = ")"
            └── body: ∅
