@ ProgramNode (location: (1,0)-(1,21))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,21))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(1,21))
            ├── flags: ignore_visibility
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── name: :f
            ├── message_loc: (1,0)-(1,1) = "f"
            ├── opening_loc: ∅
            ├── arguments: ∅
            ├── closing_loc: ∅
            └── block:
                @ BlockNode (location: (1,2)-(1,21))
                ├── locals: [:a, :b, :c]
                ├── parameters:
                │   @ BlockParametersNode (location: (1,4)-(1,19))
                │   ├── parameters:
                │   │   @ ParametersNode (location: (1,5)-(1,18))
                │   │   ├── requireds: (length: 0)
                │   │   ├── optionals: (length: 1)
                │   │   │   └── @ OptionalParameterNode (location: (1,5)-(1,11))
                │   │   │       ├── flags: ∅
                │   │   │       ├── name: :a
                │   │   │       ├── name_loc: (1,5)-(1,6) = "a"
                │   │   │       ├── operator_loc: (1,7)-(1,8) = "="
                │   │   │       └── value:
                │   │   │           @ IntegerNode (location: (1,9)-(1,11))
                │   │   │           ├── flags: decimal
                │   │   │           └── value: 42
                │   │   ├── rest: ∅
                │   │   ├── posts: (length: 1)
                │   │   │   └── @ RequiredParameterNode (location: (1,13)-(1,14))
                │   │   │       ├── flags: ∅
                │   │   │       └── name: :b
                │   │   ├── keywords: (length: 0)
                │   │   ├── keyword_rest: ∅
                │   │   └── block:
                │   │       @ BlockParameterNode (location: (1,16)-(1,18))
                │   │       ├── flags: ∅
                │   │       ├── name: :c
                │   │       ├── name_loc: (1,17)-(1,18) = "c"
                │   │       └── operator_loc: (1,16)-(1,17) = "&"
                │   ├── locals: (length: 0)
                │   ├── opening_loc: (1,4)-(1,5) = "|"
                │   └── closing_loc: (1,18)-(1,19) = "|"
                ├── body: ∅
                ├── opening_loc: (1,2)-(1,3) = "{"
                └── closing_loc: (1,20)-(1,21) = "}"