@ ProgramNode (location: (0...26))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...26))
    └── body: (length: 1)
        └── @ CallNode (location: (0...26))
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── message_loc: (0...1) = "f"
            ├── opening_loc: (1...2) = "("
            ├── arguments:
            │   @ ArgumentsNode (location: (2...3))
            │   └── arguments: (length: 1)
            │       └── @ CallNode (location: (2...3))
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── message_loc: (2...3) = "a"
            │           ├── opening_loc: ∅
            │           ├── arguments: ∅
            │           ├── closing_loc: ∅
            │           ├── block: ∅
            │           ├── flags: variable_call
            │           └── name: "a"
            ├── closing_loc: (3...4) = ")"
            ├── block:
            │   @ BlockNode (location: (5...26))
            │   ├── locals: [:x, :y]
            │   ├── parameters:
            │   │   @ BlockParametersNode (location: (8...14))
            │   │   ├── parameters:
            │   │   │   @ ParametersNode (location: (9...13))
            │   │   │   ├── requireds: (length: 2)
            │   │   │   │   ├── @ RequiredParameterNode (location: (9...10))
            │   │   │   │   │   └── name: :x
            │   │   │   │   └── @ RequiredParameterNode (location: (12...13))
            │   │   │   │       └── name: :y
            │   │   │   ├── optionals: (length: 0)
            │   │   │   ├── rest: ∅
            │   │   │   ├── posts: (length: 0)
            │   │   │   ├── keywords: (length: 0)
            │   │   │   ├── keyword_rest: ∅
            │   │   │   └── block: ∅
            │   │   ├── locals: (length: 0)
            │   │   ├── opening_loc: (8...9) = "|"
            │   │   └── closing_loc: (13...14) = "|"
            │   ├── body:
            │   │   @ StatementsNode (location: (17...22))
            │   │   └── body: (length: 1)
            │   │       └── @ CallNode (location: (17...22))
            │   │           ├── receiver:
            │   │           │   @ LocalVariableReadNode (location: (17...18))
            │   │           │   ├── name: :x
            │   │           │   └── depth: 0
            │   │           ├── call_operator_loc: ∅
            │   │           ├── message_loc: (19...20) = "+"
            │   │           ├── opening_loc: ∅
            │   │           ├── arguments:
            │   │           │   @ ArgumentsNode (location: (21...22))
            │   │           │   └── arguments: (length: 1)
            │   │           │       └── @ LocalVariableReadNode (location: (21...22))
            │   │           │           ├── name: :y
            │   │           │           └── depth: 0
            │   │           ├── closing_loc: ∅
            │   │           ├── block: ∅
            │   │           ├── flags: ∅
            │   │           └── name: "+"
            │   ├── opening_loc: (5...7) = "do"
            │   └── closing_loc: (23...26) = "end"
            ├── flags: ∅
            └── name: "f"
