@ ProgramNode (location: (1,0)-(3,24))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(3,24))
    └── body: (length: 2)
        ├── @ DefNode (location: (1,0)-(1,18))
        │   ├── name: :f
        │   ├── name_loc: (1,4)-(1,5) = "f"
        │   ├── receiver: ∅
        │   ├── parameters:
        │   │   @ ParametersNode (location: (1,6)-(1,13))
        │   │   ├── requireds: (length: 0)
        │   │   ├── optionals: (length: 1)
        │   │   │   └── @ OptionalParameterNode (location: (1,6)-(1,13))
        │   │   │       ├── flags: ∅
        │   │   │       ├── name: :foo
        │   │   │       ├── name_loc: (1,6)-(1,9) = "foo"
        │   │   │       ├── operator_loc: (1,10)-(1,11) = "="
        │   │   │       └── value:
        │   │   │           @ IntegerNode (location: (1,12)-(1,13))
        │   │   │           ├── flags: decimal
        │   │   │           └── value: 1
        │   │   ├── rest: ∅
        │   │   ├── posts: (length: 0)
        │   │   ├── keywords: (length: 0)
        │   │   ├── keyword_rest: ∅
        │   │   └── block: ∅
        │   ├── body: ∅
        │   ├── locals: [:foo]
        │   ├── def_keyword_loc: (1,0)-(1,3) = "def"
        │   ├── operator_loc: ∅
        │   ├── lparen_loc: ∅
        │   ├── rparen_loc: ∅
        │   ├── equal_loc: ∅
        │   └── end_keyword_loc: (1,15)-(1,18) = "end"
        └── @ DefNode (location: (3,0)-(3,24))
            ├── name: :f
            ├── name_loc: (3,4)-(3,5) = "f"
            ├── receiver: ∅
            ├── parameters:
            │   @ ParametersNode (location: (3,6)-(3,18))
            │   ├── requireds: (length: 0)
            │   ├── optionals: (length: 2)
            │   │   ├── @ OptionalParameterNode (location: (3,6)-(3,11))
            │   │   │   ├── flags: ∅
            │   │   │   ├── name: :foo
            │   │   │   ├── name_loc: (3,6)-(3,9) = "foo"
            │   │   │   ├── operator_loc: (3,9)-(3,10) = "="
            │   │   │   └── value:
            │   │   │       @ IntegerNode (location: (3,10)-(3,11))
            │   │   │       ├── flags: decimal
            │   │   │       └── value: 1
            │   │   └── @ OptionalParameterNode (location: (3,13)-(3,18))
            │   │       ├── flags: ∅
            │   │       ├── name: :bar
            │   │       ├── name_loc: (3,13)-(3,16) = "bar"
            │   │       ├── operator_loc: (3,16)-(3,17) = "="
            │   │       └── value:
            │   │           @ IntegerNode (location: (3,17)-(3,18))
            │   │           ├── flags: decimal
            │   │           └── value: 2
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body: ∅
            ├── locals: [:foo, :bar]
            ├── def_keyword_loc: (3,0)-(3,3) = "def"
            ├── operator_loc: ∅
            ├── lparen_loc: (3,5)-(3,6) = "("
            ├── rparen_loc: (3,18)-(3,19) = ")"
            ├── equal_loc: ∅
            └── end_keyword_loc: (3,21)-(3,24) = "end"