@ ProgramNode (location: (1,0)-(1,13))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,13))
    └── body: (length: 1)
        └── @ LambdaNode (location: (1,0)-(1,13))
            ├── locals: [:k]
            ├── operator_loc: (1,0)-(1,2) = "->"
            ├── opening_loc: (1,10)-(1,11) = "{"
            ├── closing_loc: (1,12)-(1,13) = "}"
            ├── parameters:
            │   @ BlockParametersNode (location: (1,3)-(1,9))
            │   ├── parameters:
            │   │   @ ParametersNode (location: (1,4)-(1,8))
            │   │   ├── requireds: (length: 0)
            │   │   ├── optionals: (length: 0)
            │   │   ├── rest: ∅
            │   │   ├── posts: (length: 0)
            │   │   ├── keywords: (length: 1)
            │   │   │   └── @ OptionalKeywordParameterNode (location: (1,4)-(1,8))
            │   │   │       ├── flags: ∅
            │   │   │       ├── name: :k
            │   │   │       ├── name_loc: (1,4)-(1,6) = "k:"
            │   │   │       └── value:
            │   │   │           @ IntegerNode (location: (1,6)-(1,8))
            │   │   │           ├── flags: decimal
            │   │   │           └── value: 42
            │   │   ├── keyword_rest: ∅
            │   │   └── block: ∅
            │   ├── locals: (length: 0)
            │   ├── opening_loc: (1,3)-(1,4) = "("
            │   └── closing_loc: (1,8)-(1,9) = ")"
            └── body: ∅