@ ProgramNode (location: (1,0)-(3,3))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(3,3))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(3,3))
            ├── flags: ignore_visibility
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── name: :f
            ├── message_loc: (1,0)-(1,1) = "f"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (1,2)-(1,3))
            │   ├── flags: ∅
            │   └── arguments: (length: 1)
            │       └── @ CallNode (location: (1,2)-(1,3))
            │           ├── flags: variable_call, ignore_visibility
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── name: :a
            │           ├── message_loc: (1,2)-(1,3) = "a"
            │           ├── opening_loc: ∅
            │           ├── arguments: ∅
            │           ├── closing_loc: ∅
            │           └── block: ∅
            ├── closing_loc: ∅
            └── block:
                @ BlockNode (location: (1,4)-(3,3))
                ├── locals: [:x, :y]
                ├── parameters:
                │   @ BlockParametersNode (location: (1,7)-(1,13))
                │   ├── parameters:
                │   │   @ ParametersNode (location: (1,8)-(1,12))
                │   │   ├── requireds: (length: 2)
                │   │   │   ├── @ RequiredParameterNode (location: (1,8)-(1,9))
                │   │   │   │   ├── flags: ∅
                │   │   │   │   └── name: :x
                │   │   │   └── @ RequiredParameterNode (location: (1,11)-(1,12))
                │   │   │       ├── flags: ∅
                │   │   │       └── name: :y
                │   │   ├── optionals: (length: 0)
                │   │   ├── rest: ∅
                │   │   ├── posts: (length: 0)
                │   │   ├── keywords: (length: 0)
                │   │   ├── keyword_rest: ∅
                │   │   └── block: ∅
                │   ├── locals: (length: 0)
                │   ├── opening_loc: (1,7)-(1,8) = "|"
                │   └── closing_loc: (1,12)-(1,13) = "|"
                ├── body:
                │   @ StatementsNode (location: (2,2)-(2,7))
                │   └── body: (length: 1)
                │       └── @ CallNode (location: (2,2)-(2,7))
                │           ├── flags: ∅
                │           ├── receiver:
                │           │   @ LocalVariableReadNode (location: (2,2)-(2,3))
                │           │   ├── name: :x
                │           │   └── depth: 0
                │           ├── call_operator_loc: ∅
                │           ├── name: :+
                │           ├── message_loc: (2,4)-(2,5) = "+"
                │           ├── opening_loc: ∅
                │           ├── arguments:
                │           │   @ ArgumentsNode (location: (2,6)-(2,7))
                │           │   ├── flags: ∅
                │           │   └── arguments: (length: 1)
                │           │       └── @ LocalVariableReadNode (location: (2,6)-(2,7))
                │           │           ├── name: :y
                │           │           └── depth: 0
                │           ├── closing_loc: ∅
                │           └── block: ∅
                ├── opening_loc: (1,4)-(1,6) = "do"
                └── closing_loc: (3,0)-(3,3) = "end"
