@ ProgramNode (location: (1,0)-(1,19))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,19))
    └── body: (length: 1)
        └── @ DefNode (location: (1,0)-(1,19))
            ├── name: :f
            ├── name_loc: (1,4)-(1,5) = "f"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (1,6)-(1,14))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 1)
            │   │   └── @ OptionalParameterNode (location: (1,6)-(1,11))
            │   │       ├── flags: ∅
            │   │       ├── name: :a
            │   │       ├── name_loc: (1,6)-(1,7) = "a"
            │   │       ├── operator_loc: (1,7)-(1,8) = "="
            │   │       └── value:
            │   │           @ NilNode (location: (1,8)-(1,11))
            │   ├── rest: ∅
            │   ├── posts: (length: 1)
            │   │   └── @ RequiredParameterNode (location: (1,13)-(1,14))
            │   │       ├── flags: ∅
            │   │       └── name: :b
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:a, :b]
            ├── def_keyword_loc: (1,0)-(1,3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (1,5)-(1,6) = "("
            ├── rparen_loc: (1,14)-(1,15) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (1,16)-(1,19) = "end"